FORM IMO.

REG00545-05
FOR USE IN SERVICE MANUAL:
920 & 930 WHEEL LOADERS,
REG00514

SYSTEMS OPERATION
TESTING AND ADJUSTING

920 St 930 WHEEL LOADER

AIR SYSTEM AND BRAKES
SERIAL NUMBERS
920 930
41J1-UP 41K1-UP
62K1-UP 71H1-UP
75J1-UP 73U1-UP
79J1-UP

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 INTRODUCTION

This publication has instructions and procedures for the subject on the front
cover. The information, specifications, and illustrations in this publication
are on the basis of information that was current at the time this issue was
written.

Correct operation, maintenance, test and repair procedures will give this
product a long service life. Before starting a test, repair or rebuild job, the
serviceman must read the respective sections of the Service Manual, and
know all the components he will work on.

Your safety, and the safety of others, is at all times very important. When
you see this symbol tHQ or this symbol manual, you must
know that caution is needed for the procedure next to it. The symbols are
warnings. To work safely, you must understand the job you do. Read all
instructions to know what is safe and what is not safe.

It is very important to know the weight of parts. Do not lift heavy parts by
hand. Use a hoist. Make sure heavy parts have a good stability on the ground.
A sudden fall can cause an accident. When lifting part of a machine, make
sure the machine has blocks at front and rear. Never let the machine hang on
a hoist, put blocks or stands under the weight.

When using a hoist, follow the recommendation in the manual. Use correct
lift tools as shown in illustrations to get the correct balance of the
component you lift. This makes your work safer at all times.

40400X3

NOTE: The "C" is an indication of a change from the former issue.

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 AIR SYSTEM AND BRAKES INDEX

SYSTEMS OPERATION

Air Chambers and Hydraulic Master Cylinders ....................................... 7 Electric Warning System............................................................................. 5
Air Compressor and Reservoir..................................................................... 5 Emergency and Parking Brake................................................................... 8
Air Flow.......................................................................................................... 4 Emergency and Parking Brake Control Valve ........................................... 9
Air System and Brakes ................................................................................ 4
Oil Flow......................................................................................................... 5
Brake Control Valve.................................................................................... 6 Orifice Check Valves.................................................................................... 9

Double Check Valve .................................................................................... 6 System Components..................................................................................... 5

Wheel Brake and Head Assembly .............................................................. 7

TESTING AND ADJUSTING

Air Compressor Governor.......................................................................... 10 Emergency and Parking Brake Control Valve ....................................... 11
C Air in the Hydraulic Brake System.............................................................. 15
Hydraulic Brake System Servicing........................................................... 13
Bleeding the Brakes................................................................................... 13
Brake Adjustment...................................................................................... 13 Indicator...................................................................................................... 12
Brake Control Valve................................................................................... 11
C Brake System Test ........................................................................................ 15 Leakage Check ........................................................................................... 11
Buzzer.......................................................................................................... 13
Pressure Gauge Installation...................................................................... 10
Double Check Valve .................................................................................. 12 Pressure Hydraulic System Bleeder Method .......................................... 14

Electric Warning System........................................................................... 12 Safety Relief Valve..................................................................................... 10

Emergency and Parking Brake Adjustment............................................ 11

SPECIFICATIONS

NOTE: For Specifications with illustrations, make reference to SPECIFICA­

TIONS FOR 920 & 930 AIR SYSTEM AND BRAKES, Form No.
SENR7413. If the Specifications in Form SENR7413 are not the same as in
the Systems Operation and the Testing and Adjusting, look at the printing
date on the back cover of each book. Use the Specifications in the book with
the latest date.

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 AIR SYSTEM AND BRAKES
SYSTEMS OPERATION

AIR SYSTEM AND BRAKES

SCHEMATIC OF AIR AND BRAKE SYSTEM

Doublp \ lX n S T S '°V T ? ValVe' 2' Brake diSC' 3' Brake assemblies. 4. Left service brake control valve. 5. Horn valve 6

buzzer A ?■ Right service brake control valve. 8. Starter switch. 9. Air reservoir. 10. Safety relief valve. 11 Warning
Xdraulic L^e7cZdTr'l8 F °mPreSH0r9°uern0r' 14' Air chamber' 15. Hydraulic master cylinder. 16. Air chamber. 17.
Hydraulic master cylinder. 18. Emergency and parking brake control valve. 19. Low air pressure indicator. 20 Normally-closed
pressure switch (buzzer). 21. Normally-open pressure switch (low air pressure indicator). 22. Emergency and parking bmke chambeT

The engine driven air compressor delivers air to AIR FLOW

the reservoir with the pressure controlled by the air
compressor governor. The governor is mounted on Compressed air flows from the reservoir (9) to
the compressor and causes the compressor to cut-in the two service brake control valves, (4) and (7).
and cutout as air is required. An electric warning Air flows directly through the lower chamber of
system informs the operator of low air pressure, the left control valve (4) to the horn valve (5). Air
and an emergency brake system applies the emer­ flows through the lower portion of the right
gency brake if the air pressure becomes danger­ control valve (7) to the emergency and parking
ously low.
brake control valve (18). With the air pressure in
the normal operating range and the emergency and
parking brake control valve in the RELEASED
Compressed air provides only the power to (pushed in) position, air will flow through the valve
actuate the hydraulic system of the hydraulic disc to the emergency and parking brake chamber (22)
brakes. When the service brakes are applied, the air where the air pressure compresses the spring and
chamber rods extend pushing against the hydraulic releases the emergency and parking brake. If the
master cylinder pistons. Oil flows from the master control valve is in the APPLIED (pulled out)
cylinders and applies the service brakes. position, the line to the emergency and parking

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AIR SYSTEM AND BRAKES SYSTEMS OPERATION

brake chamber is vented to atmosphere. The spring pistons causing them to push the friction pads
in the brake chamber will no longer be held against the disc.
compressed and the brake will apply.
Releasing the brake pedal relieves the air pres­
Depressing either the right or the left service sure acting against the master cylinder. The master
brake control valve pedal will apply the service cylinder piston retracts and relieves hydraulic
brakes. However, depressing the left valve will send pressure from the brake pistons. The pistons and
air through the transmission neutralizer line (1), pads are not spring returned, but merely release
releasing the transmission directional clutch. This their grip on the disc. The pads remain very close
will neutralize the transmission. Depressing the to the disc. Since the pads move only far enough
right pedal applies the service brakes only. Double away from the discs to release, no adjustment is
check valve (6) prevents air delivered by the right required to compensate for pad wear. Due to the
valve from passing through the left valve and close running clearance, the pads keep the discs
neutralizing the transmission. clean.

Air delivered by the service brake control valves

passes through a single line to the engine compart­
ment where it divides and flows to the two air
chambers (14) and (16). Two separate hydraulic
brake systems are provided as a safety feature. The ELECTRIC WARNING SYSTEM
system actuated by chamber (16) controls the
front wheel brakes while the system actuated by A buzzer and a visible indicator warn of low air
chamber (14) controls the rear wheel brakes. pressure. The buzzer and indicator switches sense
pressure in the line from the emergency and
parking brake control valve to the emergency and
OIL FLOW parking brake chamber. The pressure switch (20)
for the buzzer is closed (current flows and the
buzzer sounds) when the pressure is below 70 to
80 psi (480 to 550 kPa). The indicator pressure
switch (21) is closed (current flows and the
indicator appears green) when the pressure is above
70 to 80 psi (480 to 550 kPa). The low pressure
indicator is mounted on the dash and shows the
red side of a magnetic ball when the pressure is
low. When the pressure is high enough for safe
operation, the switch is closed and current flows to
the indicator. The current flow energizes an elec­
tromagnet and causes the ball to turn and expose
the green side.

BRAKE SYSTEM
Since the pressure switches are mounted down­
(Machines effective with 62K2779 and 41K1828) stream from the emergency brake control valve,
the buzzer will sound and the indicator will show
15. Hydraulic master cylinder. 17. Hydraulic master cylin­
der. 23. Reservoir.
red, regardless of system air pressure, whenever the
emergency brake control valve is in the APPLIED
(pulled out) position. However, with the valve
The hydraulic disc brake for each axle consists pushed in and the pressure descending, the buzzer
of the hydraulic master cylinders (15) and (17), sounds and the indicator shows red before the
connecting lines, and the brake assemblies (3). The emergency and parking brake applies. This is due
hydraulic brake fluid reservoir is an integral part of to the control valve’s lower actuating pressure.
each master cylinder. Each reservoir fluid level
must be maintained separately on machines 62K1
thru 62K2778 and 41K1 thru 41K1827. On
machines 62K2779-Up and 41K1828-Up the hy­ SYSTEM COMPONENTS
draulic brake fluid reservoirs for the master cylin­
ders (15) and (17) are connected to reservoir (23). Air Compressor and Reservoir
To prevent an air lock from developing in the
master cylinders, the fluid must always be visible in The air compressor is mounted on the engine
the reservoir (23). When the brake pedal is pushed, and driven by the engine timing gears. The air
the air chamber rod extends and pushes the master compressor governor controls operating pressure. It
cylinder piston. Piston movement creates brake is mounted on the compressor and can be adjusted
line pressure which is transmitted to the disc brake to raise or lower the compressor operating range.

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AIR SYSTEM AND BRAKES SYSTEMS OPERATION

assembly pushes the valve (9) off inlet valve seat

(8). Pressure air from inlet port (13) passes around
the valve and out through outlet port (5). Air
travels to the double check valve, air chamber and
transmission neutralizer control valve when the left
brake treadle is depressed, or the double check
valve and air chamber when the right brake treadle
is depressed.

AIR COMPRESSOR

1. Air compressor. 2. Air compressor governor.

1. Seat. 2. Rubber spring. 3. Piston assembly. 4.

Retainer. 5. Outlet port. 6. Spring. 7. Exhaust valve
seat. 8. Inlet valve seat. 9. Valve. 10. Valve body. 11.
Valve spring. 12. Exhaust port. 13. Inlet port. 14.
Exhaust diaphragm.

When the air pressure below piston assembly (3)

becomes greater than the force exerted above the
piston, the piston lifts sufficiently to permit valve
(9) to move upward and seat. This blocks off
further supply of pressure air. Piston assembly (3)
remains seated against valve (9), preventing any
loss of air pressure through exhaust port (12).
Thus, the brake control valve is in a balanced
position and air pressure is held in the lines and air
chambers.
A safety relief valve prevents damage to the air
system if the compressor governor should fail. The If the treadle is partially raised, mechanical force
safety relief valve is mounted on the reservoir and above piston assembly (3) is decreased. The greater
opens when air pressure reaches 150 psi (1030 force below the piston assembly, exerted by
kPa). pressure air and spring (6), lifts the piston as­
sembly, unseating it from valve (9). Pressure air in
the lines and air chambers is permitted to escape
past exhaust diaphragm (14) until forces above and
below the piston are balanced. When the treadle is
Brake Control Valve
completely raised, piston assembly (3) will remain
unseated from valve (9), exhausting all pressure air
Two brake control valves are used in the braking
to atmosphere and releasing the brakes. Valve (9) is
system. The operation is the same for each valve.
held in its seat by valve spring (11).

When the brake control valve treadle is de­

pressed, a force is exerted on seat (1). The force
compresses rubber spring (2) and moves piston Double Check Valve
assembly (3) downward. The exhaust valve seat (7)
contacts and closes exhaust port (12) in valve (9). The double check valve prevents air delivered by
Continued downward movement of the piston one service brake control valve from entering the

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AIR SYSTEM AND BRAKES SYSTEMS OPERATION

delivery circuit of the other control valve.

X2049

DOUBLE CHECK VALVE

Air from the right service brake control valve

passes into one side of the double check valve and
air from the left service brake control valve passes
into the other side. Air entering from either
AIR CHAMBERS AND HYDRAULIC MASTER CYLINDERS
control valve passes out through one delivery port.
1. Air chamber for front brakes. 2. Master cylinder for front
brakes. 3. Air chamber for rear brakes. 4. Master cylinder
Applying the left control valve sends air to the
for rear brakes.
transmission neutralizer and into the double check
valve. Air pushes the shuttle against the port
leading to the right control valve, sealing that port,
CAUTION: Always bleed air from the hydraulic
and flows to the air chambers. brake system when a line has been disconnected.
Applying the right brake moves the shuttle
against the left port, blocking air flow to the Wheel Brake and Head Assembly
transmission neutralizer valve and allows the air to
flow only to air chambers.

Air Chambers and Hydraulic Master Cylinders

The two air chambers actuate the pistons of the

hydraulic brake master cylinders. Compressed air
in the brake chamber acts against the diaphragm
and extends the rod. When the rod extends, it
pushes the hydraulic master cylinder piston. A
spring returns the diaphragm when the air pressure
is released.

The hydraulic brake fluid reservoir is an integral

part of each master cylinder. With the master
cylinder piston retracted, oil flows from the
reservoir into the cylinder. When the piston moves
it covers the port from the reservoirs and traps the
fluid in the cylinder. The trapped oil is pushed out
of the cylinder and into the brake lines.

The brake fluid displaced by the master cylinder

piston causes the brake pistons to extend and
DISC BRAKE COMPONENTS
apply the brakes. When the apply force is released,
a spring returns the master cylinder piston and the 1. Head assembly. 2. Anchor pin. 3. Friction pad. 4.
brake fluid returns to the reservoir. Metal backing. 5. Piston. 6. Disc.

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AIR SYSTEM AND BRAKES SYSTEMS OPERATION

Hydraulic pressure in the head assembly acts on

pistons to force friction pads against discs to
provide wheel braking. The discs revolve with the
wheel hubs and the head is mounted on the axle
flange. The head assembly for each wheel on the
930 Wheel Loader has one friction pad assembly
and two pistons on each side of each disc. The
head assembly for each wheel on the 920 Wheel
Loader has one friction pad assembly and one
piston on each side of each disc. The friction pads
are bonded to a metal backing. The friction pad
assemblies are held in place by anchor pins.

CAUTION: To prevent damage to pistons and

seals, do not apply brakes when brake pads are
removed.

When the brakes are applied, hydraulic pressure

forces the pistons against the pads. The pads move
out against the discs to produce braking for the
machine. 7. Bleed valves.

Emergency and Parking Brake

WARNING: The emergency and parking

A brake prevents only the drive shaft from
turning. Therefore, differential action will
allow the machine to articulate with the parking
brake applied. Always place the safety bar in
LOCKED POSITION before working beneath ma­
chine.

(920 Head Assembly is similar, but has only two pistons)

1. Head assembly. 2. Anchor pin. 3. Friction pad. 4.

Metal backing. 5. Piston (four). 7. Bleed valves. 8. Line.
9. Piston seal. 10. Anchor retaining bolt.

When the brakes are applied, the hydraulic

pressure balances the pistons and the force exerted
on each side of the disc is equal. The pistons do
not have return springs.
The pads may be replaced without removing the
head by removing the anchor pin and sliding the
pad out.
CAUTION: Do not apply brakes with pads re­
moved. The pistons must not be allowed to extend
out of their bores with the pads removed. Open the
bleed valves on the head to relieve any pressure
acting on the pistons. If the pistons should extend EMERGENCY AND PARKING BRAKE
and expose the seals, head removal will be required CHAMBER AND DRUM
to reinstall the pistons.
1. Brake chamber. 2. Brake drum. 3. Drive shaft.

Two bleed valves on each head are used to bleed

air from the hydraulic brake system. Bleed the
brakes whenever a line is disconnected in the The emergency and parking brake prevents the
hydraulic brake circuit. drive shaft from turning whenever air pressure to

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 AIR SYSTEM AND BRAKES
SYSTEMS OPERATION

the emergency brake chamber is below a safe

operating level. A brake drum with expanding
shoes is mounted on the drive shaft.

The air brake chamber actuates the expanding

shoes. The brakes are spring applied and air
released. The spring force retracts the brake cham­
ber rod pulling the brake lever UP to APPLY the
brake. Air pressure acts on the diaphragm to
compress the spring, extend the brake chamber
rod, and push the brake lever DOWN to RELEASE
the brake.

Emergency and Parking Brake Control Valve

FLOW OF AIR WITH PARKING BRAKES RELEASED
The dash-mounted emergency and parking brake
valve controls the supply of air to the emergency 1. Knob and pin assembly. 2. Valve body. 3. Spring. 4.
and parking brake chamber. With the valve pulled Orifice. 5. Outlet port (to brake chambers). 6. Inlet port
out, the line to the emergency parking brake (from reservoir). 7. Exhaust port (to atmosphere). 8. Valve
assembly.
chamber is vented to atmosphere. With the valve
pushed in, air flows from the reservoir into the
emergency and parking brake chamber and releases In the event of an air system failure or if the
the brake. pressure drops below 40 + 5 psi (280 ± 35 kPa)
spring (3) will unseat valve (8) exhausting pressure
air to the atmosphere and will apply the emergency
and parking brake.

'——If ORIFICE CHECK VALVES

The check valves are used in the air lines

between the left brake control valve and double
check valves. The check valves allow the transmis­
sion to engage before the brakes fully release. The
brake release delay is caused by the restricting
orifice in the valve (4).

FLOW OF AIR WITH PARKING BRAKES APPLIED

1. Knob and pin assembly. 2. Valve body. 3. Spring. 4.

Orifice. 5. Outlet port (to emergency brake chamber). 6.
Inlet port (from air reservoir). 7. Exhaust port (to atmos­
phere). 8. Valve assembly.

With the air reservoir discharged, the emergency

and parking brake control valve will be in the
APPLIED position. [Knob (1) pulled out.] Valve
(8) will be seated against upper face of valve body
(2) sealing off pressure air supply to outlet port
(5), and exhaust port (7). Air pressure in the
reservoir is allowed to build to the operating range.

When the air pressure gauge registers in the

operating range, the operator must push in and 1. Body. 2. Seat. 3. Spring. 4. Valve. 5. Cap.
hold knob (1) on the emergency and parking brake
control valve to release the parking brake. Pressure
air from the reservoir enters the valve through inlet
port (6), passes through orifice (4), and outlet port
(5) to the brake chamber, releasing the parking
brake. Air pressure then holds valve (8) seated over
exhaust port (7).

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AIR SYSTEM AND BRAKES TESTING AND ADJUSTING

AIR SYSTEM AND BRAKES

Tools Needed: 8M2885 0 to 200 psi (0.0 to 1380 kPa) Remove the bleed valve and install the pressure
Pressure Gauge, 5P3100 Pump Group, and gauge and hose. Leave the gauge installed for all
8F24 Hose Assembly. pressure tests.

WARNING: The emergency and parking AIR COMPRESSOR GOVERNOR

A brake prevents only the drive shaft from
turning. The differential will allow the Start the engine and allow the air pressure to
machine to articulate with the parking brake build up to governor cutout pressure. Note the
applied. Always place the safety bar in LOCKED pressure reading on the test gauge.
POSITION before working beneath machine.
If the cutout pressure (on 920) is not 122 ± 2
CAUTION: To prevent damage to pistons and psi (840 ± 14 kPa) or, if the cutout pressure (on
seals, do not apply brakes when brake pads are 930) is not 105 ± 2 psi (725 ± 14 kPa), adjust the
removed. air compressor governor as follows:

PRESSURE GAUGE INSTALLATION Loosen the locknut of the adjusting screw. Turn
the adjusting screw as required. One clockwise turn
Exhaust the air pressure from reservoir by of the adjusting screw raises the cutout pressure
depressing the cap on the bleed valve. approximately 20 psi (140 kPa).

NOTE: Cut-in pressure is automatic after the

cutout adjustment is made.

AIR COMPRESSOR GOVERNOR ADJUSTMENT

X2044
NOTE: On earlier machines, remove the panel on
BLEED VALVE the side of the engine compartment and the cover
on bottom side of the governor.

SAFETY RELIEF VALVE

T88834-B

Exhaust air pressure from the reservoir and

remove the safety relief valve. Connect valve to a
GAUGE CONNECTED
5P3100 Pump Group. Apply the pressure on the

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 AIR SYSTEM AND BRAKES
TESTING AND ADJUSTING

valve. The valve should open at 150 psi (1030 kPa). emergency and parking brake does not prevent
Adjust the valve by loosening the locknut and machine movement at engine full-load speed, make
turning the adjusting screw. an adjustment as follows. Stop the engine and
move the emergency and parking brake control
knob to the OFF position. Remove pin (4), loosen
locknut (1), turn yoke (2) to shorten the rod
assembly. Check brake operation and, if necessary,
make an adjustment to the parking brake.

SAFETY RELIEF VALVE

EMERGENCY AND PARKING

BRAKE CONTROL VALVE
BRAKE ADJUSTMENT
Start the engine and allow the air pressure to
1. Locknut. 2. Yoke. 3. Rod. 4. Pin. 5. Lever.
build up to cutout pressure. Push the emergency
brake in to release the emergency and parking
brake. Shutoff the engine and relieve the air WARNING: If the buzzer begins to sound
pressure by repeatedly applying the service brakes.
Look at the test gauge pressure reading to make A while the linkage is being adjusted, stop the
adjustment and start the engine to build
sure the emergency and parking brake control the air pressure back up. The buzzer begins
knob moves to the APPLIED (pulled out) position sounding at 70 to 80 psi (480 to 550 kPa). If the
at 40 ± 5 psi (280 ± 35 kPa). If not, recondition system pressure were to continue to bleed down to
the valve. approximately 45 psi (310 kPa), the brake will
apply.

Block the wheels and stop the engine. Measure

the movement of rod assembly (3) when the
emergency brake control knob is moved from the
APPLIED position to the OFF position. Rod
assembly movement should be .75 in. (19 mm)
minimum.

BRAKE CONTROL VALVE

Start the engine and allow the air pressure to

build up. Depress either brake treadle and hold the
treadle in the depressed position.
EMERGENCY AND PARKING BRAKE CONTROL VALVE
Release the brake treadle and depress the other
treadle. The service brake should function the
EMERGENCY AND PARKING same. Failure of front and/or rear service brakes to
BRAKE ADJUSTMENT release indicates faulty brake control valves.

With the emergency and parking brake control Leakage Check

in the brake APPLIED position, start the engine
and engage the transmission in second gear. If the Depress either brake pedal and hold it in the

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AIR SYSTEM AND BRAKES TESTING AND ADJUSTING

depressed position. Coat air line connections at the NOTE: Before beginning any test of the electric
air chambers with soap suds and check for air warning system, check all electrical connections
leakage. Release the brake pedal. Coat the exhaust and grounds to assure good contact.
port of each brake control valve with soap suds.
Leakage must not exceed a 1 in. (25,4 mm) soap Start the diesel engine and allow the air system
bubble in one minute. Correct any air leaks that pressure to build up to cutout pressure. Push the
may exist. emergency and parking brake control valve in. Shut
off the diesel engine. Turn the disconnect switch
DOUBLE CHECK VALVE ON.
Slowly exhaust the air pressure by depressing
Test the double check valve with the engine
the service brake pedal.. Note the air pressure
running, the powershift transmission in 3rd speed
reading on the test gauge at which the buzzer
forward, and the machine moving. Depress the
begins to sound and the low air pressure indicator
right service brake control valve and the engine
changes from green to red. Both should act
rpm should decrease. Depress the left service brake
between 70 and 80 psi (480 to 550 kPa).
control valve and the engine rpm should increase
due to the action of the transmission neutralizer. If
the transmission neutralizes when the right service
brake control valve is depressed, the double check Indicator
valve is defective. Install a new valve.

DOUBLE CHECK VALVE

1. Double check valve. 2. Service brake control valve. 3.

Stop light switch.

BRAKE CONTROLS

ELECTRIC WARNING SYSTEM 1. Indicator. 2. Emergency and parking brake control

valve. 3. Left brake control valve. 4. Right brake control
valve.

SCHEMATIC OF ELECTRIC WARNING SYSTEM

WARNING SYSTEM PRESSURE SWITCHES
1. Starter switch. 2. Wire to starting motor. 3. Ammeter. 4.
Low air pressure indicator. 5. Warning buzzer. 6. Normally 2. Emergency and parking brake control valve. 5. Line to
closed pressure switch (buzzer). 7. Normally open pressure emergency and parking brake chamber. 6. Buzzer
switch (low air pressure indicator). switch. 7. Indicator switch.

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AIR SYSTEM AND BRAKES TESTING AND ADJUSTING

The low air pressure indicator switch should

open and the indicator should appear red. If the
low pressure indicator remains green, the indicator
may be sticking or the switch may not be opening.
If current flows to the indicator, install a new
switch. If no current flows and the indicator
continues to show green, install a new indicator.

Buzzer

If the buzzer does not begin to sound in the

correct pressure range, check continuity of the
normally closed pressure switch. The switch should
be closed. If the switch is good, check the buzzer.
If either is defective install a new component.

DISC BRAKE

1. Disc. 2. Lining assembly.

HYDRAULIC BRAKE SYSTEM SERVICING

Use heavy duty hydraulic brake fluids meeting Bleeding The Brakes
SAE J1703c specifications only. Other fluids may
cause rubber parts to deteriorate or swell. Whenever air has entered the hydraulic brake
system, or a master cylinder has become dry, the
Avoid brake fluid contamination when servicing system must be bled. This can be done manually or
the hydraulic brake system. Most commercial parts by using a pressure hydraulic system bleeder.
cleaners may be used to clean master cylinder and
wheel cylinders if the parts are then thoroughly
rinsed with denatured alcohol or brake fluid to
remove all traces of the solvent. After removing
solvent, dry parts and protect from dust until
cylinders are reassembled. Wash the rubber parts in
clean denatured alcohol or brake fluid.

CAUTION: Do not use mineral base cleaning

solvent such as gasoline, kerosene, distillant, car­
bon tetrachloride, acetone, paint thinner, etc.
These solvents deteriorate rubber parts causing
them to become soft, tacky and swollen.

Brake Adjustment

Hydraulic disc brakes require no adjustment.

There is no return mechanism to force the lining
assembly away from the discs; therefore, the lining
assemblies remain in slight contact with the discs
to keep them clean.
MASTER CYLINDERS LOCATION
Check for minimum permissible thickness in
1. Master cylinder for front wheel brake. 2. Master cylinder
area where disc (1) and lining assembly (2) make for rear wheel brake.
contact. Apply service brake and hold in the
engaged position. Inspect discs and lining assem­
blies for wear. Minimum permissible thickness of Bleed the front wheel brake system which is
disc (1) is .450 in. (11.43 mm). New disc thickness actuated by master cylinder (1) and rear wheel
is .500 in. (12.7 mm). Minimum permissible brake system which is actuated by master cylinder
thickness of lining (2) is .125 in. (3.18 mm). New ( 2) .
lining thickness is .625 in. (15.88 mm). If the
lining thickness is measured across the complete 1. Check the fluid level in the reservoir. The
carrier and lining assembly, the minimum total initial level should be within .5 in. (12 mm)
thickness is .375 in. (9.53 mm). from the top of the reservoir.

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AIR SYSTEM AND BRAKES TESTING AND ADJUSTING

NOTE: To prevent an air lock from developing in 8. Repeat the procedure for the other wheel
the master cylinders, fluid must always be visible in brakes if necessary.
the reservoir while the system is being bled.
Pressure Hydraulic System Bleeder Method

A commercially available pressure hydraulic

system bleeder can be used. Before using the
pressure hydraulic system bleeder, be sure tank (5)
contains a sufficient quantity of brake fluid of the
recommended type and grade. Maintain a pressure
of 45 to 60 psi (310 to 415 kPa) in the tank while
bleeding the system.

On earlier machines each system is pressure bled

separately. Bleed one brake system by pressurizing
master cylinder (1) and then repeat the procedure
for the other brake system by pressurizing master
cylinder (6). Either system may be bled first.
CHECKING RESERVOIR FLUID LEVEL

On later machines equipped with the reservoirs

2. Attach a hose to the bleed valve. Place loose connected to both master cylinders, both systems
end of hose into a jar containing enough fluid can be pressurized at the same time by connecting
to submerge the end of the hose. the adapter (2) to the reservoir filler opening.

NOTE: Disc brakes have two bleed valves for each 1. Remove the plug from the master cylinder
wheel. Both valves must be opened to remove all of reservoir and install adapter (2).
the air from the brake system.
2. Attach the hose (3) to the adapter and open
the valve (4).

WHEEL BRAKE BLEED VALVES

(920 Wheel Loader illustrated)

3. Repeatedly depress the brake pedal until the

brake fluid escaping through the bleed line
contains no air bubbles.
PRESSURE HYDRAULIC SYSTEM BLEEDER
(Typical example)
4. Hold the brake pedal down and observe the
stroke indicator rod. 1. Master cylinder. 2. Adapter. 3. Hose. 4. Valve. 5.
Tank. 6. Master cylinder.

5. Open the bleed valve and observe the brake

fluid escaping from the bleed line. Close the
bleed valve when the stroke indicator rod is 3. Attach a bleed hose to the bleed valve on the
extended 1.44 in. (36.6 mm). wheel brake head. Insert the other end of the
hose into a jar containing enough brake fluid
6. Repeat Steps 3, 4, and 5 until the brake fluid to submerge the end of the hose, and open the
escaping through the bleed line contains no air bleed valve approximately one-half turn.
bubbles.
4. When the fluid flows into the jar without air
7. Refill the reservoir to within .5 in. (12 mm) bubbles, close the bleed valve and remove the
from the top of the reservoir. hose.

14
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 AIR SYSTEM AND BRAKES TESTING AND ADJUSTING

5. Bleed the cylinder on the other wheel. (1) on each master cylinder. Brake repair is needed
when the stroke indicator moves more than 1.00
6. Disconnect the pressure hydraulic system in. (25.4 mm) from the air chamber of the master
bleeder from the master cylinder, remove the cylinder.
adapter, and install the plug.

7. Repeat procedure for other brake system.

C AIR IN THE HYDRAULIC BRAKE SYSTEM

1. Stroke indicator.

1. Start the diesel engine and let the air pressure

in the reservoir go up to cutout pressure.

2. Push the brake pedal down and keep it in that

position.
MASTER CYLINDERS AND AIR CHAMBERS

1. Stroke indicators. 3. Look at the stroke indicator.

a. Slow movement of the stroke indicator

Air in the hydraulic brake system cannot be after it has stopped once, is an indication
detected by brake pedal “feel.” However, a stroke that there is leakage in the hydraulic
indicator from the air chambers on each master section of the brake system or that the
cylinder will provide a means of detecting a “soft” cup is cut.
pedal condition which will cause increased master
cylinder piston movement. If the movement of b. Extra travel of the stroke indicator (with
stroke indicators (1) is too much, check for brake linings in contact with discs) is an
hydraulic leaks or air in the system. See BRAKE indication that air is in the hydraulic
CONTROL VALVE, LEAKAGE TEST. section of the system.

NOTE: If the leak results from a damaged cup or 4. Release the brake pedal.
seal, the hydraulic brake system may be contami­
nated. Drain the brake fluid and flush the system a. If either one or both of the stroke
with denatured alcohol. If the system is free of indicators do not retract, this is an indica­
leaks, check for air. Bleed each hydraulic brake tion that a bypass opening in one of the
system. See BLEEDING THE BRAKES. master cylinders is closed by dirt, corro­
sion or the primary cup has become too
large.
C BRAKE SYSTEM TEST
NOTE: A primary cup which has become too large
Check of Operation is an indication of wrong or dirty hydraulic fluid in
the system. If hydraulic fluid is dirty, remove and
The stroke indicator (1) will give an indication repair all components in the hydraulic system of
when brake repair is needed. With the brake pedal the brakes. Flush the brake lines with clean
pushed down, make note of the stroke of the rod hydraulic fluid.

15

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 CATERPILLAR
Caterpillar, Cat and ffl are Trademarks of Caterpillar Tractor Co.

JUNE 1978
FORM NO. REG00545-05 PRINTED IN U.S.A.

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