7

Air Brakes

What Youll Learn

After reading this chapter you will be able to:
identify the components of an air brake system
explain how an S-cam foundation brake works
describe what happens when one or more air brake system
components fail
explain how trailer brakes are applied

Why Air Brakes?

Air brakes are used on heavy vehicles for a number of reasons:

Air brake systems use a much greater force to apply the brakes
than hydraulic braking systems do. This greater force is needed
to cope with the heavy loads of commercial vehicles.

Air brake systems are more tolerant to small leaks. The smallest
leak in a hydraulic system could result in brake failure. An
air brake system includes a compressor to generate more
compressed air as needed.

Air brakes are used on heavy vehicles because they have proven
they are capable of stopping these vehicles safely.

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Basic Air Brake Components

The basic components of a simple
air brake system showing the
brakes applied.

This diagram shows the components that are used to make the simplest
possible air brake system:

154

A compressor to pump air, with a governor to control the

compressor.

Air lines to allow the pressurized air to flow between the air
brake system components.

A reservoir to store the compressed air.

A brake pedal (usually called a foot valve) to apply the brakes

by directing compressed air from the reservoir to the brakes.

Foundation brakes, including brake chambers, slack adjusters,

brake linings and drums or rotors, to transfer the force
generated by the compressed air through a mechanical linkage
to apply the brakes.

Air Brakes

Air Brake Chamber Components

Diagram of a typical clamp-type air

brake chamber.

The above diagram shows the most common device used to apply truck
air brakes the air brake chamber. It converts the force of compressed
air into a strong mechanical force through the pushrod and slack
adjuster.
The air brake chamber consists of a flexible diaphragm clamped between
two steel housings. The diaphragm construction is similar to a tire
sidewall, consisting of a reinforced fabric core with a rubber coating.
Other main parts are the pushrod and plate assembly, and a return
spring.

Long stroke and regular stroke brake chambers

Many new air brake systems are equipped with long stroke brake
chambers. As the name implies, a long stroke chamber design has a
longer pushrod stroke than the pushrod of a standard brake chamber.
Long stroke brake chambers can usually be identified by square-shaped
inlet ports or a nametag on a clamp bolt.

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Chapter 7

Air brake chamber air pressure applied

The force of the compressed air

against the diaphragm causes the
pushrod to extend out of the brake
chamber.

This diagram shows how air under pressure is admitted to one side of
the diaphragm, causing it to inflate. As it inflates, the diaphragm pushes
against the pushrod, plate assembly and the return spring, causing them
to move. Note the position of the slack adjuster it is now at about a 90
degree angle to the pushrod.
The amount of pushrod force is governed by the air pressure (in pounds
per square inch) and the effective surface area of the diaphragm
(in square inches). The pushrod force is exerted against the brake
mechanism, causing the brakes to apply.
The most common size air chamber used on truck drive axles and trailer
axles is a regular Type 30 clamp type chamber with 30 square inches of
effective diaphragm area.
Air chambers are very powerful. The common Type 30 regular chamber
shown in the diagram above if applied with air pressure of 100 p.s.i.
(690 kPa) develops a pushrod force of 3,000 pounds.
Air chambers are made in a number of sizes, ranging from Type 9 (with
nine square inches of effective diaphragm area) to Type 36 (with 36
square inches of effective diaphragm area). The range of sizes allows the
truck engineer to match air chamber force with axle capacity so that no
axle is under or over braked.
However, even though truck air brake system pressures are 100 p.s.i.
(690 kPa) and above, much lower air application pressure, usually less
than 20 p.s.i. (138 kPa) is used to make normal stops.

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Air Brakes

Definition

Foundation brakes: S-cam type

Foundation Brake refers

to the mechanical parts
of the brake system
inside the wheel.

The brake assembly at each wheel is generally called the foundation

brake. The assembly consists of the brake parts around the wheel that are
operated by the air brake system, including the brake chamber. The most
popular type of foundation brake is the S cam drum brake.

When air pressure is applied to

the brake chamber diaphragm,
it causes the pushrod to move
against the slack adjuster arm
which rotates the S-cam to apply
the brakes.

This diagram shows the main components used in the S-cam drum
foundation brake. The air brake chamber pushrod is connected to a lever
arm called a slack adjuster. The slack adjuster is attached to a camshaft
with an S-shaped head called an S-cam. Air pressure applied to the
chamber causes the pushrod to move forward, causing the slack adjuster
to rotate the S-cam. This causes the brake linings to press against the
brake drum, causing friction, which causes the wheel to decelerate,
stopping the vehicle.
The slack adjuster is also the means of adjusting the brakes to
compensate for brake lining and brake drum wear. Brake adjustment is
important and is covered in Chapter 8 Air Brake Adjustment.
Brake shoe return springs are used to keep the brake linings away from
the drum when the air pressure is released from the air chamber.
The picture on the left shows the
brake linings mounted on the axle
housing. The brake drum on the
right is mounted on the wheel and
rotates with it.

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Compressor
The first requirement of an air brake system is a means of compressing air
and storing it in reservoirs (tanks) so that it is available for instant use.
The source of the compressed air is the compressor, which takes in air
from the atmosphere and compresses (pressurizes) it. The compressed air
is then pumped through an air line to a supply reservoir.
A gear-driven compressor and
governor.

RoadSense Tip
Check belt tension by
pressing down on the
belt midway between the
pulleys. If you can press
it in more than double
the width of the belt,
the tension needs to be
adjusted.

The compressor is mounted on the engine of the bus or truck. On most

new engines, the compressor is mounted on the side of the engine and
driven by gears. A belt, like a fan belt, drives some compressors. As long
as the engine is running, the compressor will be running.
All trucks use piston-type air compressors. They may have one, two
or four cylinders depending on the volume demands of the particular
vehicle.
When air is compressed, its temperature rises. With a truck air
compressor operating at a pressure of 120 p.s.i. (827 kPa), the air
temperature as it leaves the compressor is over 204 C (400 F).
To prevent the compressor from overheating, two types of cooling
systems are used. The most common method on heavy trucks is
to circulate engine coolant through the compressor, while some
compressors on lighter units may be air-cooled.

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Air Brakes

Oil is used for lubricating the moving parts of the compressor, just like
oil is used to lubricate the moving parts of a cars engine. Oil also helps
to cool the compressor. The compressor is usually lubricated from the
same oil as the engine of the truck or bus, though some compressors
have their own separate oil supply. It is important to check that there is
sufficient oil supply.
Diagram of an air compressor.

Since the compressor pumps air, it needs a supply of clean air to work
properly. Air from the atmosphere supplies both the truck engine and the
compressor. An air filter is used to keep this supply clean. The air filter
should be checked regularly to make sure it is not clogged, as this would
restrict air flow.
A piston-type compressor operates on a similar principle to that of the
intake and compression strokes of a typical car engine.

Intake stroke
As the piston moves down in the cylinder, it creates a lower pressure
(vacuum) within the cylinder than the atmospheric pressure outside the
compressor. With the inlet valve open, air is then drawn into the cylinder
to fill the vacuum.

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Air enters the compressor cylinder

on the intake stroke. On the
compression stroke, the inlet valve
is closed and the discharge valve
is open to feed the compressed air
toward the air reservoir.

Fast Fact

Compression stroke

Many current air brake

systems operate with
a maximum pressure of
approximately 125 p.s.i.
(862 kPa).

When the piston reaches the bottom of the cylinder it then begins to rise.
The inlet valve closes, causing the air in the cylinder to compress. As the
piston nears the top of the stroke, the discharge valve opens, and the
pressurized air is forced past the valve and into the discharge line leading
to the reservoir.

Governor
Fast Fact
Vehicle safety standards
require that the governor
must be set to restart
the compressor if the air
pressure drops below 80
p.s.i. (552 kPa).

The compressor is capable of compressing air to over 500 p.s.i.

(3,448 kPa). This is far higher than is needed to operate an air brake
system. Most current air brake systems operate with a maximum pressure
of 125 p.s.i. (862 kPa).
There needs to be a way to stop compressing air once a certain air
pressure has been reached. And, if the air pressure in the tanks drops
below a certain level (such as after a series of brake applications), there
needs to be a way to start compressing air again.
This is the job of the governor. When sufficient pressure has been built
up, the governor causes the compressor to go into an unloading stage.

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Air Brakes

Fast Fact
The compressor must
be capable of building
pressure in the reservoirs
from 50 p.s.i. to 90
p.s.i. within three
minutes at a fast idle
(1,000 1,200 r.p.m.).

The governor causes the unloader

piston in the compressor to open
the inlet valve.

Governors are usually set to unload the compressor (stop the compressor
from pumping air) when the air pressure reaches about
125 p.s.i. Although the maximum pressure on different vehicles may
vary between 105 and 135 p.s.i. (724 and 931 kPa), the range between
minimum and maximum pressure should be approximately 20 p.s.i.
(138 kPa).
For example, if the maximum air pressure was 125 p.s.i., the governor
would restart the compressor if air pressure in the reservoirs dropped to
105 p.s.i. (applying the brakes several times would likely cause the air
pressure to drop to this level). At any rate, the governor must restart the
compressor if the air pressure drops below 80-85 p.s.i. (552-586 kPa).

Reservoirs
Fast Fact

Steel tanks (known as reservoirs) are used to store the compressed air
from the compressor.

The safety valve is

sometimes called a popoff valve.

Air reservoir, showing safety valve

on top and drain cock on the
bottom.

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RoadSense Tip
When draining reservoirs
dont just briefly open
and close the drain
cocks. Allow plenty
of time for the air to
completely drain.

Warning
Compressed air can be
dangerous if it escapes,
particularly to eyes and
to ears.
Avoid being in the direct
path of compressed air,
such as when draining
the reservoirs.

A safety valve will be installed on the first reservoir to protect the

reservoirs from being over-pressurized and bursting if the governor was
to fail to unload the compressor. The safety valve consists of a springloaded ball to allow reservoir air to exhaust into the atmosphere. The
valves pressure setting is determined by the force of the spring. Safety
valves are normally set to vent the excess pressure at approximately
150 p.s.i. (1,034 kPa).
If the safety valve has to relieve the pressure, this means that the
governor or compressor needs service or repair. Only a qualified
mechanic should do this.
The air that is delivered from the compressor usually contains some
water vapour that condenses into liquid water. This is why the supply
reservoir is often called the wet tank. Most compressors also pass a small
amount of oil and carbon particles. The oil and any other contaminants
mix with the water, making a grey sludge.
If allowed to accumulate, this sludge would enter other components of
the braking system. An excess of water in the system causes trouble with
valves and other parts. In winter, water in the system may freeze, causing
malfunction of valves or brake chambers.
To prevent this sludge from contaminating the air valves in the
system, drain valves (also known as drain cocks) are installed in all
reservoirs. Draining the reservoirs can prevent this sludge build up. Most
manufacturers recommend that reservoirs be drained daily.

Foot Valve
Pressing on the brake pedal (called the foot valve treadle) applies the air
brakes, just like stepping on the brake pedal applies the brakes in a car.

A foot brake valve.

Fast Fact
The maximum brake
application will not
exceed the pressure in
the reservoirs.
For example, if reservoir
pressure is 80 p.s.i. (552
kPa), the maximum brake
application you could
make would not exceed
80 p.s.i.

162

The treadle (pedal) of a foot valve has a springy feel that is quite different
from the feel of a hydraulic brake pedal of a car. For one thing, you really
dont have to press harder on a foot valve to apply more braking force
you simply have to press it down a bit farther. As long as the foot valve
is held in one position, the air pressure delivered to the brake system will
remain constant.

Air Brakes

Releasing the foot valve allows the application air to be exhausted

through the assemblys exhaust ports to the atmosphere.
In effect, it is a foot-controlled pressure regulator. It is the device that
allows you to select any application pressure needed to make a gentle, or
a very rapid stop.
A unique feature of a foot control valve is the ability to maintain the
application pressure that you have chosen, even if there are small leaks
downstream from the foot valve. You need only to maintain the treadle
position and the foot valve will momentarily open, replenish any air that
has been lost, and then close all automatically.

How Air Brakes Work

Brakes applied

The driver has depressed the foot

valve to apply the brakes.

In this simplified diagram, air at full system pressure is indicated by the

dark shading in the line connecting the supply reservoir to the foot valve.
The driver is making a brake application. This can be seen by the light
shading in the air lines connecting the foot valve to the air chambers.
Arrows show the direction of air flow.
The air chambers are pressurized and the brake linings have contacted
the brake drums, slowing the vehicle.

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Brakes released

When the driver releases the

foot valve, air pressure in the
brake chambers exhausts into the
atmosphere.

In this simplified diagram, the drivers foot is off the brake pedal,
allowing the brakes to release. This action has caused an exhaust port in
the bottom of the foot valve to open, allowing the air that was applied
to the brake chambers to escape. Note the burst of exhaust air below the
foot valve.
The return springs in the air chambers have returned the pushrod
assembly to the released position, and the slack adjusters and S-cams
have rotated to their released position.
Brake shoe return springs (not shown) have retracted the brake linings
away from the brake drums.

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Air Brakes

Dual Air Brake Systems

Dual air brake systems have been in use since the mid-1970s.

A dual air brake system with the

brakes applied.

The device that made dual systems possible is the dual foot valve. It is
actually two control valves operated by a single pedal. This allows the
brake system to be divided into two completely independent sections.
Each section has its own supply, delivery and exhaust ports.
Fast Fact
Cars have had dual
braking systems since
1968 to reduce the
chance of a total brake
failure.

The two sections of the dual foot valve are named primary and
secondary. The primary section is located closest to the pedal, and in
many systems operates the drive axle brakes. The secondary section
usually operates the steering axle brakes.
When the driver applies the brakes, both sections of the dual foot valve
are activated. Air from the primary tank is applied to the rear axle brakes
and air from the secondary tank is applied to the front axle brakes.
Most dual systems use three reservoirs, a supply reservoir as before, and
two service reservoirs, one for each section of the dual system. Each
service reservoir is filled through a one-way check valve, and there are
two reservoir pressure gauges, one for each service reservoir.
Even in the event of a total failure in one or the other system, the driver
is able to make a controlled stop, using only the foot valve, although
maximum braking power will be reduced.
There are other ways of splitting a dual air brake system, but however
it is divided, if one of the systems fail, the driver is still able to make a
controlled stop.
Note the change in terminology for the reservoirs. The first reservoir (wet
tank) is called the supply reservoir. The two service reservoirs are

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called the primary reservoir and secondary reservoir, indicating the

section of the dual foot valve that they supply.
Some dual systems have the low air warning device connected to the
supply reservoir as shown, while others have two separate connections,
one located on each service reservoir.

Components of a Dual Air Brake System

Supply, primary and secondary reservoirs
Fast Fact
Some reservoirs
have more than one
compartment (and more
than one drain cock).

The compressed air from the compressor contains several contaminants

including water vapour, oil mist and carbon particles. Most contaminants
settle in the supply reservoir. Primary and secondary reservoirs have been
added so that all the air brake components, with the exception of the
governor valve, are supplied with cleaner air.

One-way check valve

One-way check valves allow air to flow from the supply reservoir to the
primary and secondary reservoirs. As the name implies, a one-way check
valve allows air to flow in one direction only. This is so that in the event
of a failure in the air compressor, compressor discharge line, or supply
reservoir, the air supply in the primary and secondary reservoirs would
not flow backward and be lost.

Reservoir pressure gauges

All air brake-equipped vehicles have at least one air pressure gauge on
the instrument panel to indicate the air pressure in the service reservoir
system.
The air pressure gauge on the
left has a single needle. The air
pressure gauge on the right has
two needles one indicating
pressure in the primary reservoir,
the other showing pressure in the
secondary reservoir. The gauge on
the left shows pressure in p.s.i.
The gauge on the right shows both
p.s.i. and kPa.

RoadSense Tip
Check gauges frequently
to make sure there is
adequate air pressure to
apply the brakes.

166

Rather than having two separate reservoir gauges, many vehicles have a
single gauge with two needles, indicating the pressure in the primary and
secondary reservoirs.
As well, many vehicles have an additional gauge to indicate how much
air pressure is being applied when the foot valve is depressed.
The reservoir pressure gauge is mounted in the dashboard so that the
status of the air brake system can be monitored while driving and during
a pre-trip inspection.

Air Brakes

Low air warning device

All vehicles equipped with air brakes must have a warning device to
indicate if the air pressure in the system drops to a dangerous level. This
could occur if there is an air leak, or if you apply the brakes repeatedly
and have used up the air supply more rapidly than the compressor can
replenish it.

Many vehicles have two low-air

warning devices a warning
indicator light on the dashboard
and a buzzer.

Fast Fact

The low air warning device must come on when air pressure drops
below 60 p.s.i. (414 kPa).

The low air warning

device must activate
when air pressure is less
than 60 p.s.i. (414 kPa).

A typical low air warning device is a red warning light on the dashboard.
There may also be a buzzer.

A wig-wag.

RoadSense Tip
If the low air warning
comes on, pull over and
stop. Do not proceed
until you find out why
the low air warning
came on and until you
determine that the air
brake system is safe.

Some older vehicles are equipped with a low air warning device near the
top of the windshield that drops into the drivers view when air pressure
drops below approximately 60 p.s.i. This type
of warning device is known as a wig-wag.
Some wig-wags automatically retract when
air pressure rises above the warning level of
60 p.s.i.; some wig-wags need to be manually
pushed up to the out of view position after
the air pressure has risen above the warning
level.
When a low air warning device activates,
stop the vehicle and find the cause of the air
loss. The air pressure remaining in the system
(approximately 60 p.s.i.) will be enough to stop
the vehicle if you act promptly.

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Quick release valve

In the previous diagram, when the driver released the brakes, all the air
contained in the air lines and in the air chambers was vented through the
foot valve exhaust port. Because of the distance that the exhaust air has
to travel, there can be a considerable lag time for the brakes to release.
This is where the quick release valve comes in.
A quick release valve allows the brakes to release quickly and fully, by
allowing the pressurized air to exhaust near the brake chambers. In this
diagram a quick release valve is placed close to the front brake chambers
between the foot valve and the air chambers.
A quick release valve has been
installed between the front brake
chambers and the foot valve.

When the brakes are applied, air from the foot valve flows through the
quick release valve to the chambers in the normal manner.
When the driver releases the foot valve, only the air in the line between
the foot valve and the quick release valve is vented at the foot valve
exhaust port. The larger volume of air contained in the air chambers is
vented at the exhaust port of the quick release valve.
Note the difference in the air bursts at the foot valve and at the quick
release valve there is a much bigger burst of exhausting air at the quick
release valve.
Quick release valves may be found in a number of places in an air brake
system, including front brakes, rear brakes, spring parking brakes and any
other place that the rapid exhausting of air is required.

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Air Brakes

Relay valve
A relay valve has been installed between the reservoir and the rear
brake chambers.

Relay valves are used to reduce the lag time when the brakes are applied,
and when they are released. They are remote controlled air valves that
respond to a control signal from the foot valve. They are usually mounted
on a frame rail close to the air chambers that they are to operate.
Relay valves are supplied with air directly from the primary or secondary
reservoirs through a large diameter air line (shown as the supply line
in the diagram) so that there is a high volume of air available for rapid
delivery to the air chambers.
The pressure of the reservoir air delivered in this way will be the same as
the control pressure delivered by the foot valve. If you make a 20 p.s.i.
(138 kPa) brake application, approximately 20 p.s.i. of air pressure would
be directed to the rear brake chambers through the relay valve.
When the driver releases the foot valve, only the air in the control line is
vented at the foot valve exhaust port. The volume of air contained in the
air chambers is vented through an exhaust port built into the relay valve.
Relay valves are designed to handle the volume requirements of two or
four air chambers. Though they are primarily found on rear axle brakes,
relay valves are sometimes found on steering axle brakes or wherever
there is a need to apply and release air rapidly.
For simplicity, quick release valves and relay valves are not shown in
the following diagrams because they do not change the basic concept of
an air brake system, but serve only to speed up the release of the brakes,
if needed.

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Dual System with Primary System Failure

This diagram shows the worst case failure where a line rupture has
caused a total loss of pressure in the primary reservoir.

Despite a loss of primary reservoir

pressure, the front (secondary)
brakes will still operate.

Air pressure in the secondary reservoir has been protected by the

one-way check valve. The low air warning system must activate when
pressure in any reservoir falls below 60 p.s.i. (414 kPa) to alert the driver
to the problem. In many systems, the warning will come on at pressure
above 60 p.s.i.
When you apply the brakes, you will be able to make a controlled stop,
but only the steering axle brakes will apply, and thus stopping distances
will be longer because the braking force will be reduced.
If the failure had been in the secondary system, braking on the rear axle
would have been maintained, but the steering axle brakes would not
operate.
The compressor will continue to pump air, but all of its output will take
the path of least resistance and be vented at the line rupture.
If the low air warning system activates at any time, stop immediately and
do not proceed until a repair has been made.

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Air Brakes

Parking Brakes
While air pressure does an excellent job in helping to stop a vehicle by
applying the foundation brakes, it is totally unreliable (and illegal) for
parking. If a vehicle were to be parked using only the air brakes, any
leaks in the system, or any failure in a hose, diaphragm, or air valve
would result in loss of air pressure and a possible rollaway collision.
Regulations for parking brakes require that once applied, the parking
force must be maintained by mechanical means and be unaffected by loss
of air pressure.
The most common type of parking brake in an air brake system is the
spring parking brake. The second type is known as a safety actuator and
is usually found only on some highway coaches and intercity buses.

Spring Parking Brakes

Most spring parking brakes consist of an additional chamber attached
to the rear of a service brake chamber. The added chamber contains a
powerful coil spring arranged so that the spring force can be applied to
the brakes through the normal service chamber pushrod.

A spring parking brake chamber

attached to a service brake
chamber. The brakes are off.

This diagram shows the main components of a typical combination

spring and service brake chamber.

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Fast Fact
While the spring parking
brakes will apply if air
pressure falls beneath
a certain level, their
effectiveness in stopping
the vehicle or holding a
parked vehicle depends
on how well the brakes
have been kept in
adjustment.

Spring parking brakes are mounted on the rear axles only not on
steering axles. The service brake chamber contains the normal pushrod,
diaphragm and return spring. The spring parking brake section is
mounted behind the service brake chamber.

This concrete mixer has spring

brakes on its tandem rear axles
only not on the twin steering
axles, nor on the booster axle at
the rear.

The spring parking brake chamber contains a second diaphragm, a large

coil spring, and an intermediate pushrod that is used to transmit the force
of the coil spring to the service brake pushrod when the spring parking
brake is applied. The coil spring in most spring parking brake chambers
can exert a force of between 1,500 and 2,000 pounds.

Warning
The spring in a spring
parking brake is very
powerful. If the spring
were ever to break out
of the chamber, it could
cause serious damage to
any objects or persons
in its way.
Keep away from a spring
parking brake chamber
that shows any sign of
damage, such as dents
or cracks anywhere in
the housing.

172

When you make a regular foot brake application, air pressure is applied
against the diaphragm in a service brake chamber, causing the diaphragm
to inflate, pushing the push rod out against the slack adjuster to apply the
foundation brakes.
Spring parking brakes work in the opposite way. These brakes are applied
and remain applied by mechanical spring pressure, not by air pressure.
If air pressure falls beneath the amount needed to keep the spring
compressed, the spring pushes against the push rod in the service brake
chamber, pushing the push rod out against the slack adjuster to apply the
foundation brakes (because the parking brake chambers are piggy-backed
onto the service brake chambers and operate the foundation brakes
through the same linkage).
Spring parking brake assemblies should only be serviced by qualified
personnel. The spring in a spring parking brake chamber is under
extreme pressure and could cause serious personal injury.

Air Brakes

Applying and releasing spring parking brakes

There are several ways to apply and release spring brakes.
RoadSense Tip
In some systems, spring
parking brakes will
gradually apply as air
pressure goes down,
rather than apply all at
once. In some of these
systems, they may begin
to apply at 90 p.s.i. or
higher.

Normally they are applied and released by using the parking

brake control valve on the dashboard.

If the air pressure in the system falls below approximately 60

p.s.i., the spring brakes may begin to drag, and at 20 to 45 p.s.i.
(138 to 310 kPa) may automatically fully apply.

Spring parking brake released

Air pressure in the spring parking

brake chamber keeps the spring
parking brake off. There is no
air pressure in the service brake
chamber.

This diagram shows a spring parking brake chamber in the released

position. The service brake is also in the released position.
Air at reservoir pressure has been supplied to the spring parking brake
section. The parking brake diaphragm has inflated, compressing the main
spring. The spring parking brakes are now released.

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A parking brake control valve (usually a yellow button) is mounted on

the dashboard. In most cases, pushing this valve in allows air pressure
to flow to the spring parking brake chambers, causing these spring
parking brakes to release. Pulling this valve out exhausts the air pressure
against the spring parking brake chamber, causing these brakes to apply.
Instructions are usually imprinted on the button.
While the push-pull parking brake control is the most common, some
systems use a switch, usually set so that flipping it in one direction applies
the spring parking brakes and flipping it in the other direction releases
them.

A parking brake control valve.

Spring parking brake service brake applied

The driver has applied the service

brake. Air pressure in the spring
parking brake chamber keeps the
spring parking brakes off.

The driver has applied the foot valve, delivering air to the service brake
port, inflating the service brake diaphragm.

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Air Brakes

Spring parking brake spring brake applied

The driver has applied the spring

parking brake. There is no air
pressure in either the spring
parking brake chamber or the
service brake chamber.

The driver has placed the parking brake control valve in the park
position. This has caused the air from the spring parking brake section to
be exhausted.
The force of the coil spring has been transmitted to the intermediate
pushrod, which in turn has pushed against the service brake diaphragm,
pushrod, and slack adjuster, applying the brakes.

Driver alert compounding of brakes

Always be sure that the spring parking brakes are released before making
heavy service brake applications, such as those made during a pre-trip
inspection.

RoadSense Tip
Avoid compounding the
brakes.
Always release the spring
parking brakes before
making a heavy service
brake application.

When spring parking brakes are applied, there is up to 2,000 pounds of

force applied to all of the brake components. If a heavy service brake
application is made, the force of the air application is added to the spring
force. This could add a further 3,000 pounds for a total of 5,000 pounds
This adding together of the two forces, known as compounding can
cause damage to slack adjusters, s-cams, brake chamber mounting bolts,
brake shoe rollers, shoes and brake drums.
Note that lighter brake applications of less than 30 to 40 p.s.i. (207 to
276 kPa), to prevent a vehicle from rolling while the spring parking
brakes are being released or applied, are not harmful.

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Chapter 7

Spring parking brake manual release

Most spring parking brake chambers have a means of manually releasing,
or caging an applied spring parking brake. This feature should only be
used by mechanics when making a repair.
If all air is lost and the vehicle has to be towed, spring parking brakes can
be released by caging them. Always block the wheels when caging spring
parking brakes. Once a spring brake chamber is caged, there will be no
parking brake force at that wheel.
Some chambers have a built-in release bolt while others have a release
bolt, nut and washer carried in a bracket mounted on the chamber housing.
The spring has been caged using a
caging bolt.

RoadSense Tip
Caging spring parking
brakes should be done
only in an emergency.
Once a spring brake
chamber is caged, there
will be no parking brake
force at that wheel.

176

This diagram shows how one type of release bolt is inserted into the
rear of the spring parking brake housing. The release bolt is then given
a quarter turn to lock it in place. Then the release nut is turned until the
spring is compressed.
Instructions for manual release are usually imprinted on the housing of
most spring parking brake chambers.
Before attempting to manually release spring parking brakes, block
the wheels to prevent the vehicle from rolling. To move a vehicle after
manually releasing the spring parking brakes call a tow truck!.

Air Brakes

Spring parking brakes in dual air brake systems

This installation takes advantage of the primary and secondary reservoirs
to supply the parking brake dash control with air from the tank that has
the highest pressure.
Spring parking brakes installed in
a dual air brake system. The brakes
are off.

Fast Fact
The effectiveness of
spring parking brakes
depends on how well the
brakes have been kept in
adjustment.

This is accomplished by the use of a two-way check valve. The air that is
delivered from the two-way check valve is frequently called blended air.

Blended air
The two-way check valve draws
air from the tank with the highest
pressure.

The two-way check valve has two inlet ports and one delivery port.
A free floating shuttle within the valve will move away from the inlet that has
the higher pressure, and the higher pressure will be supplied to the parking
brake control.
This arrangement will also ensure that the spring parking brakes will
not automatically apply if there is a total loss of air pressure in either
reservoir.

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Chapter 7

Dual air brake system partial system failure

Despite the ruptured air line from
the primary reservoir, the driver
can still make a controlled stop.

This diagram shows the benefit of the blended air supply for the parking
brake system. There has been a loss of air from the primary reservoir.
The two-way check valve shuttle has moved so that secondary reservoir
pressure supplies the parking brake control valve.
The result is that the spring parking brakes do not apply automatically.
The low air warning system has alerted the driver to the air loss, allowing
the driver to make a controlled stop using the front axle brakes.
Some vehicles with dual air systems are equipped with an optional device
called a spring brake modulator. This device senses a loss of pressure
in the primary system, and when the driver applies the service brakes,
causes air to be exhausted from the spring parking brakes in direct
proportion to the brake application. By simply applying the foot valve
normally, the driver controls the amount of spring force used to assist the
front brakes to bring the vehicle to a controlled stop.
All vehicles must meet Canadian Motor Vehicle Safety Standards for
emergency stopping, so regardless of how the dual system is arranged,
or if a spring parking brake modulator is installed, the vehicle will have
adequate braking force, even with a partially failed air system.
With all systems, after stopping, the driver can securely park the vehicle
by manually applying the parking brake control valve.

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Air Brakes

Safety actuator parking brakes

Safety actuator parking brakes are used on many buses and highway
coaches. They are similar in appearance to spring parking brakes but
their operation is very different.
Rather than using a powerful coil spring, this brake uses a one-way
locking mechanism that can be engaged to allow the pushrod to
stroke outward, but prevent it from returning. The actuators have two
diaphragms, one to apply the service brake, and the other to apply the
parking brake.
Safety actuator parking brake
chambers are found on some
buses.

A separate air reservoir is used for parking and the parking brake dash
control is identical in appearance and operation to the one used for
spring parking brakes.
Pulling the dash control outward simultaneously applies air pressure to
the parking diaphragm and engages the locking mechanism. The push rod
moves out, applying the brakes. The pushrod is then held in the applied
position by the locking mechanism. The vehicle is parked securely, even if
air is lost from all reservoirs.
Normally, pushing the dash control inward causes air to exhaust from the
parking diaphragm and at the same time releases the locking mechanism,
allowing the pushrod to retract. However, if more than 4 p.s.i. (27.6 kPa)
pressure has been lost from the parking reservoir, the parking brakes will
not release. A heavy service brake application must also be made, causing
the pushrod to move slightly ahead, allowing the locking mechanism to
disengage.
Because spring force is not used for parking, safety actuator parking
brakes cannot be compounded.
Note that safety actuator parking brakes will not apply automatically,
even if service reservoir pressure is drained or pumped down to zero.
Only loss of pressure in the parking reservoir will cause automatic
application.

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Chapter 7

Tractor-Trailer Air Brake Systems

To understand the basics of tractor-trailer air brake systems, it is best to
start with the trailer. Once the trailer system is understood, it becomes
simpler to understand the components that are needed to tow a trailer.
A trailer system has many of the components found on a truck system.
Of course, the trailer must have foundation brakes, air chambers, air
reservoirs and control valves. The only major item not found on a trailer
air system is an air compressor.
The trailer system must rely on the tractor for two important needs. First,
the trailer must receive the compressed air from the tractor to fill the
trailer reservoirs. Second, the trailer system must receive the commands
from the tractor as to when to apply, and when to release the brakes.
To fulfill these needs, there are two air line connections between the
tractor and the trailer air systems.
The air line that supplies the trailer reservoirs with air at full tractor
reservoir pressure is called the supply line. It is sometimes called the
emergency line.
Fast Fact
The supply line may also
be called the emergency
line.
The control line may also
be called the service line.

180

The line that carries the control signal from the tractor is called the
control line. It is also commonly called the service line.
Because tractors and trailers need to be disconnected and reconnected
from time to time, the air lines are equipped with quick coupling devices
called glad hands. Each coupler resembles a human hand about to make
a handshake to give you the glad hand. Glad hands are often colourcoded a blue line or blue colouring on a glad hand is used to indicate
the control line, and red is used to indicate the supply line.

Air Brakes

Glad hands allow easy and quick

connection between the tractor
and trailer.

RoadSense Tip
When not in use,
glad hands should be
fastened to dead-end
(dummy) couplers or to
each other to prevent
dirt and debris from
entering the air lines.
Glad hands should
also be secured to
prevent the lines from
chafing against other
components or bouncing
off the vehicle.

There are two basic types of trailer air systems those that use spring
parking brakes and those that do not. Although most current trailers
do use spring parking brakes, there are a number of earlier trailers and
converter dollies in use that are not equipped with spring parking brakes.
All trailer systems, with spring parking brakes or without, must have an
emergency stopping system that will fully apply the trailer brakes in the
event that the trailer separates from the tractor.
Trailers that are not equipped with spring parking brakes use a device
called a relay emergency valve. If this valve senses that the trailer has
broken away from the tractor, it applies the trailer service brakes with
full trailer reservoir pressure. This action is called dynamiting the trailer
brakes.
Trailers equipped with spring parking brakes use the spring force to apply
the brakes (dynamite the trailer brakes) if the trailer breaks away from
the tractor.

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Chapter 7

Trailer with relay emergency valve charging

The diagram shows a trailer equipped with a relay emergency valve. Air
is passing from the tractor through the supply line to the relay emergency
valve, filling the trailer reservoir.
Compressed air from the tractor
flows through the supply line to fill
the trailer reservoir.

Trailer with relay emergency valve applying

This diagram shows a normal service brake application. A control signal
from the tractor has been sent through the control line to the relay
emergency valve, which reacts to this signal in exactly the same way as
the tractor relay valve previously described.
When the driver makes a brake
application, air flows through the
control line.

The relay emergency valve has drawn air from the trailer reservoir and
delivered it to the trailer service chambers at approximately the same
pressure as the control signal.
On highway trailers, one reservoir and one relay emergency valve
are used for single or tandem axles. Some tandem logging trailers are
equipped with a reservoir and a relay emergency valve for each axle.

182

Air Brakes

Trailer with relay emergency valve dynamited

This diagram shows a broken supply line. The relay emergency valve has
sensed the loss of pressure in the supply line, and has delivered full trailer
reservoir pressure to the service brake chambers, dynamiting the brakes.
The trailer brakes will remain applied as long as pressure is retained in
the trailer reservoir.
The broken supply line has caused
the trailer brakes to dynamite.

Warning
Small leaks in trailer
systems not equipped
with spring parking
brakes can cause the
applied pressure to be
reduced or depleted,
possibly allowing a
parked trailer to roll
away.
Always block the wheels
of a parked trailer to
ensure it cannot roll.

The trailer brakes will also be dynamited each time the glad hands are
disconnected, or when the driver closes the trailer supply valve that is
located on the tractor dashboard.
Motor vehicle safety standards require that these systems remain applied
for a minimum of 15 minutes.
It is important to follow proper procedures when coupling a tractor to
a parked trailer to prevent the trailer from moving and possibly causing
damage. Coupling procedures are detailed in Chapter 4 Skills for
Driving Trucks and Trailers.

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Chapter 7

Trailer with spring parking brakes charging

This diagram shows a typical trailer system that uses spring parking
brakes for parking and for emergency (breakaway) stopping.
This trailer is equipped with spring
parking brakes.

RoadSense Tip
You can usually tell if
you have reversed the
supply line and service
line connections:
the spring parking
brakes will not
release even when
you use the parking
brake control on the
dashboard

The system shown uses one reservoir and two air valves, a relay valve for
the service brakes, and a trailer spring brake valve that fills the reservoir
and controls the spring parking brakes.

the trailer service

brakes will not
function

Other systems may be equipped with one, two or three air valves and
multiple reservoirs. However, the use of more or less air valves or
additional reservoirs will not alter the basic operation of the system.

Avoid these situations by

taking care in coupling
to a trailer.

The tractor is delivering air through the supply line to the trailer
spring brake valve. The spring parking brake valve directs air to fill the
reservoir(s) and to release the spring parking brakes.
There are two types of systems:

RoadSense Tip
Always perform a tug
test after coupling the
tractor to a trailer.
Before conducting a tug
test, use a flashlight to
visually check that the
fifth wheel locking jaws
are closed and locked.

184

One type fills the reservoir(s) before releasing the spring parking
brakes.

The other type releases the spring parking brakes first, then fills
the reservoir(s).

Always perform a tug test after coupling the tractor to the trailer. Follow
the coupling procedures as shown in Chapter 4 Skills for Driving
Trucks and Trailers.

Air Brakes

Trailer with spring parking brakes service brake application

In the diagram, a control signal from the tractor has been sent through
the control line to the trailers relay valve. The relay valve has drawn
air from the trailer reservoir and delivered it to the trailer service brake
chambers at approximately the same pressure as the control signal.
The trailer brakes have been
applied by pressing on either the
foot valve or applying the hand
valve, or both.

Trailer with spring parking brakes dynamited

This diagram shows a broken supply line. The trailer spring brake valve
has sensed the loss of pressure in the supply line and has exhausted the
air pressure from the spring parking brake chambers, causing the spring
parking brakes to apply. Note the burst of air from the exhaust port of the
trailer spring brake valve.
The supply line has broken, causing
the trailer spring parking brakes to
apply.

This action can also be called dynamiting of the trailer brakes. The trailer
brakes will also be dynamited each time the glad hands are disconnected,
or when the driver closes the trailer supply valve that is located on the
tractor dashboard.

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Chapter 7

Tractor protection
Fast Fact
A peace officer may
place your vehicle out
of service if the tractor
protection system allows
the tractor pressure to
drop below 20 p.s.i.
(138 kPa) without the
trailer air supply valve
automatically closing.

If the mechanical connection between the tractor and trailer were to fail,
causing the trailer to separate from the tractor, the two connecting air
lines would break. Air pressure from the tractor system would rush out
through the broken supply line, and if the driver were making a brake
application, air pressure would also rush out through the broken control
line.
To prevent the tractor air from being depleted to an unsafe level, tractors
are equipped with a tractor protection system.
A tractor protection system consists of a trailer air supply valve located
in the tractor dash, and a tractor protection valve, usually located behind
the tractor cab. All of the supply and control air delivered to the trailer
passes through the tractor protection valve.
In the event of a trailer breakaway, the tractor protection system will
automatically shut off air loss from the tractor, preserving enough
pressure for the driver to make a safe stop.
Some tractor protection systems will shut off immediately in the event
of a breakaway, but some will allow tractor pressure to drop to as low as
20 p.s.i (138 kPa) before shutting off.
Proper operation of the tractor protection system should be checked as
part of the daily pre-trip inspection.

Trailer air supply valve

Once the supply line is connected to the trailer, there needs to be a way
of directing air pressure to the trailer.

Trailer air supply valve.

This is the job of the dash mounted trailer air supply valve. It senses
air pressure in the supply line that carries air to the trailer system. Most
trailer air supply valves are an octagon-shaped red button.

186

Air Brakes

Hand valve
Applying the foot valve directs approximately the same application
pressure to both the tractor and trailer brakes. For example, if you make
a 20 p.s.i. foot valve application, this application pressure will be applied
to both the tractor and trailer brakes.
There are times when it may be beneficial to apply only the trailer brakes
without applying the tractor brakes, such as when coupling the tractor to
a trailer.
The hand valve allows independent
control of the trailer brakes. Some
are mounted on the steering
column, others are mounted in the
dashboard.

Warning
DO NOT use the hand
valve for parking.
When parked, apply the
tractor parking brakes,
close the trailer supply
valve and block the
trailer wheels.

This is the purpose of the hand valve. When the trailer air brake system
is fully connected to the tractor, the hand valve allows you to apply the
trailer brakes independently of the tractor.
The hand valve should not be used in normal or emergency situation
braking. Always use the foot valve for service braking.
Most hand valves are spring-loaded, just like the foot valve, so that when
you release it, it will return to the released position. Do not use the hand
valve for parking.

Two-way check valve

The two-way check valve allows you to apply the trailer brakes
independently. This valve is identical in construction to the one used
in spring parking brake installations, except that it allows the highest
application pressure from the hand valve or the foot valve to be directed
to the trailer brakes.

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Chapter 7

Bobtail tractors
Driving a tractor without a trailer attached is called bobtailing.
Driving a tractor without a trailer
attached is called bobtailing.

Because a bobtail tractor has very little weight over the rear drive
axles, it is very easy to lock up the rear brakes, even with a light brake
application.
To help prevent this unwanted lockup, and to increase control, some
tractors are equipped with a bobtail proportioning system.
This system consists of two special valves, one controlling the steering
axle brakes, and the other controlling the drive axle brakes.
When the tractor is being driven with a trailer attached, the tractor
brakes operate normally.
When bobtailing, the braking pressure to the drive axle brakes is reduced
by as much as 75 per cent, preventing the rear brakes from locking.
At the same time, the steering axle brakes receive full application
pressure.
A tractor with a bobtail proportioning system will stop in a shorter
distance and control will be increased, especially on wet or slippery road
surfaces.
Because the steering axle brakes are doing most of the braking, a higher
than normal pedal pressure is required.

188

Air Brakes

Dual air tractor-trailer system foot valve applied

A dual air tractor system for

towing a trailer.
Note Depending on the air
brake system configuration used,
the hand control valve may be
supplied from blended air, or from
primary or secondary reservoir
pressure.
To avoid confusion, the air supply
source to the hand valve is not
shown in this diagram or in the
diagrams on the next two pages.

This diagram shows only the two service reservoirs, the dual foot valve,
and the components that are added to a tractor with a dual air system so
that it can safely tow a trailer with air brakes.
The components added are a trailer air supply valve, tractor protection
valve, hand control valve, and a pair of two-way check valves.
Two-way check valves are installed so that whichever brake is applied
foot valve or hand valve a control signal will be sent to the trailer.
The driver is making a foot valve application. The tractor front and rear
brakes are being applied, and a control signal is being sent to the trailer
through both of the two-way check valves.
Note that in most dual systems, the parking brake control valve (yellow
button) is interlocked with the trailer supply valve (red button) so that
applying the parking brake control valve causes all of the parking brakes
on both the tractor and trailer to apply.
Some tractors are equipped with three dashboard control valves the
parking brake control valve (yellow button), the trailer supply valve (red
button), plus a tractor parking brake valve with a round blue button that
can control the tractor parking brakes independently of the trailer brakes.

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Chapter 7

Dual tractor-trailer system primary air system failure

Despite the ruptured air line from

the primary reservoir, the driver
can still make a controlled stop.

Fast Fact
A peace officer may
place a trailer out of
service if the trailer
brakes do not apply
when the trailer air
supply valve is closed.

This diagram shows a tractor with a dual air system where there has been
a failure in the primary air system on the tractor. The low air warning
would have alerted the driver to the problem and a glance at the reservoir
gauges would confirm that only one part of the dual air system had been
lost.
The driver is making a foot valve application, causing the tractor front
brakes to apply. Application air from the secondary foot valve is also
passing through both of the two-way check valves, to the trailer control
line, signalling the trailer brakes to apply.
If it were the secondary system that had failed, a foot valve application
would apply the rear tractor brakes, directing air through both of the
two-way check valves to signal the trailer brakes to apply.
The same motor vehicle safety standards that require automatic shutoff of
the air supply to the trailer in the event that the pressure in the tractor
air system is lowered to between 20 and 45 p.s.i. (138 and 310 kPa)
apply equally to tractors with dual air systems.
Because the trailer supply valve is now supplied with blended air from
a two-way check valve, the automatic shutoff will not occur until the
service reservoir with the highest pressure is lowered to between 20 and
45 p.s.i. (138 and 310 kPa).
The automatic shutoff requirement should be checked as part of a pretrip inspection. If it doesnt function properly, the vehicle must be placed
out of service until it is repaired.

190

Air Brakes

Dual tractor-trailer system trailer breakaway

Despite the ruptured supply

line, the driver can still make a
controlled stop.

This diagram shows how the tractor protection valve and the trailer air
supply valve act together to protect the tractor air supply from being
depleted to an unsafe level in the event that the trailer separates, causing
the connecting lines to rupture. The sudden loss of air through the
broken trailer supply line has caused the trailer air supply valve to shut
off automatically.
The driver is making a foot valve application, causing the tractor service
brakes to apply. The application pressure is also passing through both of
the two-way check valves to the tractor protection valve.
Because there is no pressure in the supply line to the trailer, the tractor
protection valve has closed the passage to the trailer control line, so no
application air can be wasted through that broken line.
If the control line separates, nothing will happen until the trailer brakes
are applied. When that happens, the tractor protection system will
activate to protect the tractor air supply.
When no trailer is connected, the trailer air supply valve will be in the
closed position. This allows the tractor to be driven bobtail so that no air
will be lost through the disconnected glad hand couplers.

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Chapter 7

Other types of foundation brakes

There are three other types of foundation brakes found on air braked
vehicles. These are:

wedge brakes

air disc brakes

air-over-hydraulic brakes

Wedge brakes

A typical wedge brake.

This type of brake uses one or two small air chambers with wedge-shaped
pushrods. Once quite common on drive and trailer axles, wedge brakes
are now usually found only on steering axles.
RoadSense Tip
Wedge brakes should
only be adjusted or
repaired by a qualified
mechanic.

When the brakes are applied, air pressure in the brake chamber pushes
the wedge part of the pushrod between two rollers, forcing the brake
linings out to contact the brake drum.
Most wedge brakes have internal automatic adjusters. Checking proper
adjustment requires that inspection hole covers in the backing plate
be removed so that brake linings movement can be checked while the
brakes are applied and released. If either linings move more than 1/16
of an inch, or a total of 1/8 of an inch for both linings, the automatic
adjusters have failed.
Unlike conventional s-cam braking systems, drivers cant easily check the
wedge brake adjustment of a wedge brake.
Adjustment and repairs to wedge brakes should only be carried out by a
qualified mechanic.

192

Air Brakes

Air disc brakes

An air disc brake.

RoadSense Tip
Air disc brakes should
only be adjusted or
repaired by a qualified
mechanic.

This type of brake uses a rotor, or disc, that is mounted to the wheel hub
and rotates with the wheel. Two brake pads are located on either side
of the rotor. When applied, the brake pads are pressed against the rotor.
This action is similar to that of a large C clamp.
There are a number of different linkages used between the air chamber
and the operating mechanism. This illustration only shows one type,
although the principle of the others is similar.
Most air disc brakes feature an internal automatic brake adjustment
mechanism to adjust for brake pad wear. Chamber stroke limits are the
same as for automatic slack adjusters.
Unlike conventional s-cam braking systems, drivers cant easily check the
adjustment of an air disc brake.
Adjustment and repairs to air disc brakes should only be carried out by a
qualified mechanic.

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Chapter 7

Air-over-hydraulic brakes
Air-over-hydraulic brakes are frequently found on middleweight trucks
and buses. This type of braking system combines the features of an air
brake system with that of a hydraulic braking system.
Hydraulic foundation brakes offer several advantages on commercial
vehicles of this size, including light weight, compact size and proven
automatic adjusting mechanisms.
Most middleweight commercial vehicles of this size were once powered
by gasoline engines, which supplied a source of engine vacuum so
that vacuum boosters for the hydraulic brakes could be used. The now
common diesel engine does not supply a usable vacuum, so a partial air
brake system has been adopted.
An air-over-hydraulic braking
system.

Fast Fact
An air brake endorsed
drivers licence is
required to operate most
vehicles equipped with
air-over-hydraulic brakes.
The only exception is if
the brake system does
not use an air treadle. In
this case, an air brakeendorsed drivers licence
is not required.

An air-over-hydraulic braking system, such as shown above, consists of

a compressor, governor, air storage tanks, foot valve and two air-overhydraulic pressure intensifiers. The system may also include spring
parking brakes. Like a full air brake system, typical air-over-hydraulic
braking systems use a standard air pressure of around 125 p.s.i.
(862 kPa).
A standard dual air foot valve is used. Pressing on the foot valve directs
air pressure to the air-actuated side of the hydraulic pressure intensifiers,
causing the hydraulic-actuated side of the intensifiers to direct hydraulic
pressure to the foundation brakes. In other words, air pressure actuates
the braking action, but hydraulic pressure delivers the braking force to
the foundation brakes to stop the vehicle.
To provide a parking brake, many air-over-hydraulic braking systems
have a parking brake chamber attached to the foundation brake.

194

Air Brakes

RoadSense Tip
Check gauges frequently
to make sure there is
adequate air pressure to
apply the brakes.

The parking brake is controlled by the same dashboard-mounted parking

brake control valve used on vehicles with full air brake systems. Applying
the parking brake control valve on the dashboard applies the spring in
the parking brake chamber, which forces a wedge between the brake
shoes, thereby applying the brakes. Releasing the parking brake control
valve directs air pressure to the parking brake chamber to contract the
wedge and spring.
Like a full air brake system, if there were a serious air leak in an air-overhydraulic system, eventually the brakes would cease to function properly.
For this reason, drivers need to know and understand how the system
works, and check air pressure gauges frequently.

Other Air Brake System Components

The following are other components commonly found in air brake
systems.

Air dryers
Air dryers are optional devices that are installed in the compressor
discharge line between the compressor and the first reservoir. They are
designed to remove any water vapour, oil mist and carbon particles from
the air before it is delivered to the first reservoir.
Most new air brake systems include
an air dryer.

Fast Fact
Even if the air brake
system includes an air
dryer, air reservoirs
should still be drained
regularly to check for
contaminants.

RoadSense Tip
If you notice more than
just a few drops of water
when you drain the
supply reservoir, the air
dryer or compressor may
need servicing.

The warm, moist air from the compressor enters the

dryer where a certain amount of the water vapour
condenses on cool metallic surfaces. The air then
passes through a filter that removes any oil and
through another filter that removes the remaining
water vapour. From there the clean air passes
through an internal one-way check valve, and on to
the first reservoir.
When the reservoir has come up to full pressure,
a purge port in the bottom of the air dryer will open.
The collected contaminants are ejected along with a
sudden burst of air.
At the same time, a certain amount of clean air is allowed to flow back
through the filters. This reverse flush effect cleans both filters in readiness
for the next compression cycle. The purge port remains open until the
compressor resumes pumping.
Some air dryers are equipped with an electric heating element to prevent
freezing in cold weather.
In systems with an air dryer, the safety valve is often installed at the air
dryer rather than at the supply reservoir.
Air dryer operation can be checked by periodically looking for water in
the reservoirs. More than a few drops may indicate that the air dryer or
compressor requires servicing.

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Chapter 7

Fast Fact
Even if the air brake
system includes an
alcohol evaporator or
alcohol injector, air
reservoirs must still be
drained regularly.

Alcohol evaporators and alcohol injectors

Alcohol evaporators and alcohol injectors are optional devices that
introduce a small amount of alcohol vapour into the air system. The
alcohol vapour combines with any moisture that may be present. In
effect, the alcohol acts as an anti-freeze, lowering the freezing point of
any moisture that has collected in the air system.
Alcohol evaporators are connected to the inlet side of the compressor so
that alcohol vapour is drawn in and compressed along with the intake
air, which is then carried throughout the system.
Alcohol injectors are installed in the compressor discharge line between
the compressor and the supply reservoir. The discharge air passes through
a venturi (a tube with a narrow section, which causes air flowing through
the tube to create a vacuum), picking up alcohol vapour and carrying it
throughout the system.

An alcohol evaporator bottle.

RoadSense Tip
Only use products
specifically designed for
use in air brake systems
in alcohol evaporators or
alcohol injectors.

196

The alcohol reservoir should be kept topped up with methyl hydrate

during the winter months and it is a good practice to begin before the
first freeze of the season to ensure trouble-free operation.
These systems are designed to use pure methyl hydrate to provide the
alcohol and be sure to use only methyl hydrate specifically formulated for
use in alcohol evaporators or alcohol injectors.

Air Brakes

Automatic drain valves

Automatic drain valves, sometimes called spitter valves are optional
devices installed on some or all of the reservoirs on some air brake
systems. They intermittently expel any contamination that has collected.
Most are self-contained and momentarily open each time reservoir
pressure lowers two or three p.s.i. (13.8 or 20.7 kPa), but some are
connected to the compressor governor and momentarily open each time
that the compressor cycles.
Some automatic drain valves are equipped with an electric heating
element to prevent freezing in cold weather.
All automatic drain valves incorporate a manual means of checking for
the presence of moisture in the reservoirs.
The manual drains should be opened periodically to check for the
presence of water in reservoirs.
As mentioned previously, if more than a few drops of water are found,
or if contaminants are found, the compressor or air dryer may need
servicing, or the automatic drain valve may not be functioning correctly.

Front wheel limiting systems

On some vehicles, an optional system is installed to reduce the possibility
of steering axle brake lockup and resultant loss of steering control on
slippery surfaces. There are two types of front wheel limiting systems:

automatic front wheel limiting systems

manual front wheel limiting systems

Automatic front wheel limiting systems

An automatic front wheel limiting system consists only of a limiting
valve, sometimes called a ratio valve, mounted near the steering axle.
There is no dashboard control.
At very low application pressures, no air pressure is delivered to the
steering axle brakes. As application pressure exceeds the holdback
point (five to 15 p.s.i. 34.5 to 103 kPa), limited application
pressure is delivered to the steering axle brakes. At brake application
pressures below 40 p.s.i. (276 kPa), the steering axle brake pressure is
approximately 50 per cent of drive axle pressure.
At application pressures above 40 p.s.i., the percentage gradually rises,
until at an application pressure that may be used during an emergency
stop (60 to 70 p.s.i. 414 to 483 kPa), steering axle and drive axle brakes
receive equal pressure. A built-in quick release function helps to speed up
the release of the steering axle brakes.

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Manual front wheel limiting systems

Manual front wheel limiting systems are no longer installed on new
vehicles. This type of system consists of a limiting quick-release valve
mounted near the steering axle brakes, and a dash mounted control
valve. The control valve may be a flip type switch, as shown, or a pushpull type.
A manual front wheel limiting
control.

With the control valve in the dry position,

the steering axle brakes are applied with the
same pressure as the drive axle brakes.
In the slippery position, the application
pressure to the steering axle brakes is limited
to 50 per cent of drive axle brake application.

RoadSense Tip
On vehicles equipped
with a manual front
wheel limiting valve,
keep the control in
the normal (Dry Road)
position to have normal
braking efficiency.
Under the Motor Vehicle
Act, the Slippery Road
position valve may only
be used when weather
conditions make its use
essential for safety.

Fast Fact
Spring parking brake
emergency release
systems are sometimes
called California spring
parking brake systems.

Commercial vehicle safety standards permit

reduced braking on steering axle brakes only when weather and road
surface conditions make such operation essential to safety. Tests have
shown that front wheel skids do not have as dangerous an effect as do
the drive axles locking up.
The limiting quick release valve also acts as a normal quick release valve,
helping to speed up the release of the steering axle brakes.

Spring parking brake emergency release system

This system provides a special emergency release tank that can be used to
release spring parking brakes if a disabled vehicle needs to be moved to a
safe parking area and its main reservoir pressure is lost.
A second dashboard control valve is added so that air from the
emergency release tank can be directed to the spring parking brakes to
release them. This control valve is usually a dead man type that must
be held in place while the vehicle is being moved. Once the vehicle has
been moved, the spring parking brakes are re-applied by releasing hand
pressure from the control.
Instructions for operating the emergency release system are usually found
on the control valve or on a decal on the dashboard.
The popularity of this system was reduced with the introduction of the
dual air system, but it is still sometimes used on transit buses, school
buses and fire trucks.

Pressure protection valves

Pressure protection valves are frequently installed between the service
brake reservoirs and any non-essential air-operated accessories such as
air seats, air horns, air windshield wipers, air suspensions, fifth wheel
sliders and air shifts. Some air brake systems integrate the air dryer with
the supply reservoir these also use pressure protection valves.
They are designed to cut off the air supply to these systems if a failed
accessory causes the service reservoir pressure to drop below a preset

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pressure, ensuring that sufficient pressure is maintained in the service

system so that a safe stop can be made.
Shutoff pressures vary between 60 and 90 p.s.i. (414 and 620 kPa),
depending on the manufacturers specifications.

Application pressure gauges

Some trucks and tractors are equipped with one or more optional gauges
that indicate the actual pressure being delivered to the service brakes.
There may be a single gauge or
separate gauges for tractor and trailer
brake application.
Tractors may have a single gauge that
indicates application pressure if either
the foot valve or trailer hand control
valve is applied.
Some vehicles have air pressure
gauges to indicate application
pressure as well as pressure in the
reservoirs.

Anti-lock Braking Systems

Anti-lock Braking Systems (ABS) are typically made up of three main
sections: speed sensing, decision-making, and brake releasing or
modulation.

In this diagram, vehicle speed is sensed by magnetic pickups mounted in

close proximity to toothed wheels that are attached to some or all of the
wheel hubs. As the wheels rotate, a pulsating electrical current is generated.

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This pulsating current is monitored by a simple computer called an

electronic control unit (ECU). The ECU is powered by the vehicle
electrical system. During normal brake application, if the ECU detects a
sudden change in the pulsating current, the ABS system will activate.
If the brakes are applied too hard for road conditions, and a wheel
lockup occurs, the rate of the pulsating current will rapidly decrease.
The ECU, sensing the sudden drop in wheel speed, will signal electrically
controlled solenoid air valves to release air pressure from the brake
chambers at the affected wheels. The solenoid valves are frequently called
modulators.
As the brakes begin to release, the wheels will regain traction, the
pulsating current will be restored, and the ECU will allow the brakes to
re-apply. If the lockup re-occurs, the apply-and-release cycle will repeat
as often as necessary. Most systems are capable of cycling the brakes up
to five times per second.
To achieve the shortest possible stopping distance on extremely slippery
surfaces, you simply have to apply and maintain firm continuous pressure
on the brake pedal. You need to apply the brake pedal in order to allow
the ABS system work to stop the vehicle from skidding. The ABS system
will rapidly apply and release the brakes as often as necessary. There may
be some noise and vibration. ABS prevents the axle brakes from locking
up allowing the driver to retain complete steering control.
The ABS lights for the tractor and
trailer brakes should be on when
you first start the tractor.

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RoadSense Tip
When coupling to a
trailer, always check to
see if it is equipped with
ABS.
Stopping in an emergency
with a combination
unit where the tractor
and all trailers are
ABS-equipped is quite
different than stopping
a combination where all
the units do not have
ABS.

Trucks and tractors are equipped with a dash mounted failure warning
lamp that monitors the ABS system. When the ignition switch is
first turned on, the ABS system performs a self-checking sequence.
Depending on the system, the dash lamp may light, flash briefly, then
stay lit until vehicle speed reaches 711 km/h, or light briefly, then turn
off.
If the lamp does not go out, or comes on during vehicle operation, it
is signalling that there has been a failure in the ABS system. Normal
braking is still operational, only the anti-lock feature is disabled. The
vehicle may be driven to a service depot for repairs.

Trailer ABS air brake systems

Trailer ABS systems use similar components as those on trucks and
tractors. The ECU may be powered from the stop lamp circuit, or may
have a dedicated power source through the electrical connector.
Trailers with ABS air brakes will also have an indicator visible in the
tractors mirror to indicate if the system is not functioning properly. This
warning light may be mounted on the front left side of the trailer or on
the rear left side of the trailer.
On some air brake systems, there may be a trailer ABS warning indicator
on the dashboard of the tractor.

This ABS warning light is

positioned on the side of the
trailer at the rear. The driver
should be able to see it in the
tractor side mirror.

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Review Questions

202

1.

Why are air brakes, rather than hydraulic brakes, used on

heavy commercial vehicles?

2.

What are the five components of a simple air brake system?

3.

What prevents total loss of air pressure in the service brake

system in the event of an air line rupture between the
compressor and the supply reservoir?

4.

How can you tell how much air pressure is in the main
reservoirs?

5.

What must you do if the low pressure warning indicator

activates?

6.

What is one advantage of a dual air brake system?

7.

In a dual air brake system, if an air line in the secondary

braking system ruptures, how would you know? What
would happen if you then made a brake application?

8.

How does a spring parking brake work?

9.

What are the two ways that the spring in a spring parking
brake chamber can be held in the released position?

10.

Why should you avoid compounding the brakes?

11.

Why are spring parking brakes a reliable type of parking

brake?

12.

What is the purpose of the tractor protection system on

a tractor?

13.

If a driver makes a 20-pound (138 kPa) brake application

with the hand valve, what is the application pressure at the
tractor brakes?

14.

What would happen if the control line to the trailer

becomes disconnected while you are driving the tractortrailer combination?

15.

How does a wedge brake work?

16.

How does an air-over-hydraulic braking system work?

17.

Where would an air dryer be installed in an air brake

system?

18.

If you make a full brake application during an emergency

stop with an automatic front wheel limiting system, how
much air pressure is directed to the front brakes?