What is Brake Fluid?

In our daily driving routine of our car, we tend to neglect what’s happening under the car, specifically
the braking system. For the driver, as long as the brake system is functioning like normal whenever
required, we assume that there is nothing to be worried about.

Safety in driving is very important and cannot be compromised. It is vital as humans’ lives (driver and
pasengers’) are at stake regardless of the types of vehicles we drive. Brake fluid, also known as the
hydraulic fluid, is used to transfer the fluid from our foot pedal to the brake hydraulic system. Hence,
proper care maintenance and service of brake fluid are as just important as any other fluids used in our
vehicles.

History of Brake Fluid

The first braking system used in the automobile is the mechanical brake system, however hydraulic
systems were developed relatively early on. In 1914, the first hydraulic braking system was introduced
by Frederick Duesenberg from America, and in 1918 Lockheed Corporation co-founder, Malcom
Laugheed developed their own hydraulic braking system.

The first hydraulic brake system was developed by Fred Duesenberg.

At the early stage, castor oil and alcohol based hydraulic fluids (inclusive of butanol and ethanol) were
used. These fluids were used prior to the brake fluid regulation was established and DOT 2 standard was
introduced for oil-based brake fluid. DOT 2 standard is no longer in use in the automotive industry as it
has the lowest wet and dry boiling points of all the brake fluids.

In the modern hydraulic brake fluid, petroleum and glycol based brake fluid were used and the standard
DOT 3, 4, and 5.1 describe the specifications that they have to meet. This follows by the new
development of silicon-based fluids, however they aren’t as widely used.

Why Brake Fluid is Important

There are two main important criteria when it comes to the selection of brake fluid; we should ask 2
questions:

What is the boiling point?

Is the brake fluid compatible with the braking system?

The principle concept of the braking system is that the brake fluid is used as a medium to transfer the
incompressible hydraulic fluid via the driver’s foot press on the brake pedal and transfer the fluid
pressure to the master cylinder(s) to the brake calipers in order to lock the friction material against the
discs. This pressure is multiplied by the mechanical pedal and hydraulic ratio of the master cylinders,
booster (if used) and caliper piston(s) - refer to the diagram.

Fresh brake fluids are incompressible in nature and is hygroscopic materials. It has a natural tendency to
absorb moisture from the humidity in the air. Due to this, it is not advisable to leave the brake fluid
reservoir cap off for too long than needed. Over the time, the added moisture can decrease the boiling
point and increase corrosion. Overheated brake fluid can (and will) give significant effect to the
function. It can boil in the caliper and produce bubbles within any boiling fluid. Gas is compressible,
hence, boiling brake fluid leads to the “spongy” response to the brake pedal along the driving operation.
Regularly the practice to change the brake fluid according to the OEM recommendation is important as
it can prolong the life of the braking system and also ensure the safety of the driver at maximum
condition.

Brake fluid Properties

There are two main organizations which define the quality and specification of the brake fluids; The US
Department of Transportation (DOT) and The US Federal Motor Vehicle Safety Standard (FMVSS).

The US Department of Transport (DOT) classified brake fluids into two; “dry boiling point” and “wet
boiling point’. It is rated as DOT 3, DOT 4, DOT 5 and DOT 5.1. FMVSS 116 specification under the FMVSS
do not dictate the chemical composition of a given classification, or grade of brake fluid. FMVSS 116
defines the fluid properties such as “dry and wet boiling point” (referred as the equilibrium reflux boiling
point, dry and wet), the viscosity of the brake fluid at certain temperatures, high temperature stability,
corrosion characteristics, and the effects of the fluid on seals, as well as other physical properties like
the tendency to jell or separate (called stratification) or form sludge and/or crystalline deposits. 

For consumers, the most important properties is the boiling point and fluid viscosity. For modern
vehicle braking systems, they operate with ABS and Active Handling Control systems where the viscosity
is very important factor for a proper operation.

Type of Brake fluid

Most brake fluids are available in the combination of various types of glycols, which are the mixture of
alcohol based and non-petroleum fluids. The mixing process of these chemistries based is known as
“polyglycol”. This type of brake fluids is known as the non-silicon based fluids while silicon based fluids
are considered as the high grade brake fluid. However, the application is limited due to the
compatibility with the braking system and cost. In addition, this type of fluids cannot be mixed with
other type of brake fluids.

DOT 3

Glycol ether based is the commonly used DOT 3 fluids in the brake fluid industry. It is most economical
to use and meet with the requirements. DOT 3 is widely used in the automobile industry and suitable
for passenger car as well as heavy duty vehicles.

DOT 3 fluids must have a minimum dry boiling point (measured with 0 percent water by volume) of
205°C and a minimum wet boiling point (measured with 3.7 percent water by volume) of 140°C. 
The Society of Automotive Engineers (SAE), study shows that DOT 3 can absorb 2 percent of its volume
in water every year. Over time it is enough to produce excessive moisture that will cause corrosion in
the brake system which can lead to issues like a vapor-lock or a spongy pedal.

DOT 4 

DOT 4 fluid is also made of glycol ether based, but a borate esters has been added to improve properties
including increased dry and wet boiling points. This fluid is more stable with a higher boiling point if
compared to DOT 3.

By FMVSS116 standards, DOT 4 fluids must have a minimum dry boiling point of 230°C and a minimum
wet boiling point of 155°C. DOT 4 and DOT 3 are interchangeable, however it's best to avoid adding DOT
3 fluid to a system that is already using DOT 4

DOT 5 

DOT5 brake fluid is made of silicone based (SSBF) composition.  It is often referred as the synthetic
based brake fluid, which means that it does not absorb moisture. Silicon based brake fluid was designed
for use in applications where its resistance to water absorption (and therefore low corrosion) was
required such as in military equipment.

Due to the high compressibility is required to achieve the performance characteristics, DOT 5 silicon
based fluid is less popular compared to other conventional automotive or high-performance
applications.

FMVSS 116 standard outlined that DOT 5 fluids must have a minimum dry boiling point (measured with
0 percent water by volume) of 260°C and a minimum wet boiling point (measured with 3.7 percent
water by volume) of 180°C. 

Silicon based brake fluid is not interchangeable and not compatible with the glycol based brake fluid.
DOT 5 is normally produced in purple color, which is meant to be distinguished from DOT 3, DOT 4 and
DOT 5.1 brake fluid.

DOT 5.1

Brake fluid DOT 5.1 is a non-silicon based brake fluid developed to achieve DOT 5 wet and dry boiling
point specifications and viscosity requirements. It is glycol ether based with borate to enhance the
performance especially the boiling point.

DOT 5.1 can be mixed with DOT 3 or DOT 4, and usually has the highest rated boiling point which is best
recommended for severe duty and high-performance applications.

Service and Maintenance for Brake Fluid

Over certain time of normal vehicle operation, the performance of brake fluid will be deteriorated. As
the hygroscopic materials, it can be contaminated with water, air and cause problems to the seal and
other internal brake component. The standard practice to do servicing and maintenance of the brake
fluid system are by using two processes, known as bleeding and flushing. These two processes, involve
removing some amount of brake fluid and replacing it with fresh fluid.
Brake Fluid Bleeding

Brake fluid bleeding can be performed without special equipment. However, in the situations where the
fluid is clean, but air has entered into the system, then brake system bleeding process will need to be
done manually. The steps in this process are:

1. To inspect and ensure if the brake master cylinder is full.

2. Minimum two people are needed to assist on the bleeding process where one person help to
firmly depress the brake pedal and one person to check if the fluid is filled and flushed
completely in the system. By doing so, the trapped air will be completely removed from the
system.

3. To loosen the bleeder screw on one of the calipers or wheel cylinders.

4. To catch the fluid that leaves the bleeder screw in a pan, and look for signs of air bubbles in the
stream.

5. To tighten the bleeder screw.

6. To have your helper release the brake pedal.

This process can be repeated as many times as necessary in order to remove all air from the system. If it
still doesn’t work, or the fluid is dirty, you may use a vacuum brake bleeder to accomplish the procedure
without a helper. In that case, it’s also a good idea to pump the dirty fluid out of the master cylinder first
and replace it with clean fluid.

Brake Fluid Flushing

The other method of servicing brake fluid requires a flushing device. These devices vary in design, but
they are essentially designed to force clean brake fluid through the hydraulic system in order to remove
both air and dirty fluids. The steps in brake fluid flushing include:

1. To pump and remove the dirty fluid out of the brake master cylinder reservoir.

2. To attach the flushing device to the brake master cylinder reservoir with the appropriate
adapter.

3. To activate the flushing device so that it applies pressure to the system.

4. To loosen one bleeder screw at a time, catching the fluid in a pan and watching for signs of air
and dirty fluid.

5. After each wheel has been bled, shut off the flushing device and test the brake pedal.

Some vehicles have special flushing procedures, in which case you may have to bleed the wheels in a
specific order or activate a valve in order to remove all of the air from the system. In any case, it is
imperative that you verify the proper operation of the brakes before reassembling and test driving the
vehicle.

Low Brake Fluid

Some vehicles have a warning light on the instrument cluster that indicates low brake fluid, and others
require regular visual inspection of the master cylinder reservoir. In either case, low brake fluid can
indicate one of two things. In most cases, low fluid simply indicates that the brakes are wearing
normally, which results in displacement of the pistons in the calipers and more “space” in the hydraulic
system. If that is the case, then you should simply add the appropriate type of brake fluid in order to top
off the reservoir.

Some vehicles have a brake fluid level indicator.

If low brake fluid is accompanied by a pedal that feels soft, fades, or sinks to the floor, then you may
have brake system problems. In that case, you will need to perform a visual inspection of all the
hydraulic system components to check for leaks, or take your vehicle in to a professional automotive
technician for inspection.

PETRONAS Brake fluid

PETRONAS has two types of brake fluids which are PETRONAS Brake and Clutch Fluid DOT 3 and are
PETRONAS Brake and Clutch Fluid DOT 4. It is colorless and specially formulated and tested for the
automotive hydraulic brake and clutch control system where DOT 3 and DOT 4 fluid is recommended.

PETRONAS Brake and Clutch Fluid DOT 3 and DOT 4 is suitable for the application in all brake disc
(including ABS) brake drum and hydraulic clutch system meeting to the FMVSS 116 DOT 3 and DOT 4
performance level.

Other than meeting the requirement of the DOT and FMVSS 116 standard, PETRONAS Brake and Clutch
Fluid DOT 3 and DOT 4 also meet the following specification; SAE JI703, ISO 4925,MS 119 and JIS K2233.

It is recommended to change brake fluid regularly for safety and prolong the component service life. In
practice, brake fluid should be changed within 1-2 years or 20,000KM – 40,000 KM. However, the
changing frequency varies from model to model depending on the manufacturer recommendation.

Prepared by Ir Mohamad Surif Abdul Wahab, Nov 2017