Automotive brake fluid has many responsibilities. Corrosion protection and
lubrication of brake system components are only a portion of the role brake
fluid must play.

All automobiles that have a hydraulic braking system must use brake fluid in
order for the brake system to operate. The type of fluid used can depend on
the type of vehicle and the demands of the vehicles brake system.

The two most common brake fluids used in the automotive industry are fluids
that contain Polyalkylene Glycol Ether and fluid that contains Silicone or
Silicium-based Polymer. Both Fluids are common but very different in regards
to the manner in which they perform. Fluids containing Polyalklene Glycol
Ether are more widely used and are the only fluids that should be used in
racing brake systems.

Because brake systems may reach extreme temperatures brake fluid must have
the ability to withstand these temperatures and not degrade rapidly.

Fluids containing Silicone are generally used in military type vehicles and
because Silicone based fluids will not damage painted surfaces they are also
somewhat common in show cars.

Silicone-based fluids are regarded as DOT 5 fluids. They are highly
compressible and can give the driver a feeling of a spongy pedal. The higher
the brake system temperature the more the compressibility of the fluid and
this increases the feeling of a spongy pedal.

Silicone based fluids are non-hydroscopic meaning that they will not absorb
or mix with water. When water is present in the brake system it will create
a water/fluid/water/fluid situation. Because water boils at approximately
212º F, the ability of the brake system to operate correctly decreases, and
the steam created from boiling water adds air to the system. It is important
to remember that water may be present in any brake system. Therefore
silicone brake fluid lacks the ability to deal with moisture and will
dramatically decrease a brake systems performance.

Fluids containing Poly glycol ethers are regarded as DOT 3, 4, and DOT 5.1.
These type fluids are hydroscopic meaning they have an ability to mix with
water and still perform adequately. However, water will drastically reduce
the boiling point of fluid. In a passenger car this is not an issue. In a
racecar it is a major issue because as the boiling point decreases the
performance ability of the fluid also decreases.

Poly glycol type fluids are 2 times less compressible than silicone type
fluids, even when heated. Less compressibility of brake fluid will increase
pedal feel. Changing fluid on a regular basis will greatly increase the
performance of the brake system.

All brake fluids must meet federal standard #116. Under
this standard is three Department of Transportation (DOT) minimal
specifications for brake fluid. They are DOT 3, DOT 4, and DOT 5.1 (for
fluids based with Polyalkylene Glycol Ether) and DOT 5 (for Silicone based

MINIMAL boiling points for these specifications are as follows:


Dry Boiling Point Wet Boiling Point
DOT 3 401ºF 284º F

DOT 4 446º F 311º F
DOT 5 500º F 356º F
DOT 5.1 518º F 375º F




Racing brake fluids always exceeds the DOT specifications for dry boiling
points. Wet boiling points generally remain the same.

DOT 3 VS. DOT 4 and 5.1
AFCO's 570º brake fluid is a DOT 3 type fluid. However, it has a dry boiling
point that is 52º higher than DOT 5.1 specifications, 124º higher than DOT 4
specifications and 169º higher than DOT 3 specifications. AFCO's 570º fluid
meets or exceeds all DOT 3, 4, and 5.1 lubrication, corrosion protection and
viscosity specifications.

AFCO's 570º racing fluid meets but does not exceed federal standards for wet
boiling point specification; therefore, its classification is DOT 3. Because
AFCO's 570º fluid is intended for use in racing type brake systems that
undergo frequent fluid changes, exceeding federal standards for wet boiling
points is of little concern. Racing brake fluids always exceeds the DOT
specifications for dry boiling points. Wet boiling points generally remain
the same.

The term boiling point when used regarding brake fluid means the
temperatures that brake fluid will begin to boil.

The minimum temperatures that brake fluids will begin to boil when the brake
system contains 3% water by volume of the system.

The temperatures that brake fluid will boil with no water present in the

Water/moisture can be found in nearly all brake systems. Moisture enters the
brake system in several ways. One of the more common ways is from using old
or pre-opened fluid. Keep in mind, that brake fluid draws in moisture from
the surrounding air. Tightly sealing brake fluid bottles and not storing
them for long periods of time will help keep moisture out. When changing or
bleeding brake fluid always replace master cylinder caps as soon as possible
to prevent moisture from entering into the master cylinder. Condensation,
(small moisture droplets) can form in lines and calipers. As caliper and
line temperatures heat up and then cool repeatedly, condensation occurs,
leaving behind an increase in moisture/water. Over time the moisture becomes
trapped in the internal sections of calipers, lines, master cylinders, etc.
When this water reaches 212º F the water turns to steam. Many times air in
the brake system is a result of water that has turned to steam. The build up
of steam will create air pressure in the system, sometimes to the point that
enough pressure is created to push caliper pistons into the brake pad. This
will create brake drag as the rotor and pads make contact and can also
create more heat in the system. Diffusion is another way in that
water/moisture may enter the system.

Diffusion occurs when over time moisture enters through rubber brake hoses.
The use of hoses made from EPDM materials
(Ethlene-Propylene-Diene-Materials) will reduce the amount of diffusion OR
use steel braided brake hose with a non-rubber sleeve (usually Teflon) to
greatly reduce the diffusion process.

Brake fluids dry boiling point is more important then wet boiling point when
used in a racing brake system.

Passenger cars very rarely will undergo a brake fluid change making the wet
boiling point more important.

Racing brake system fluid is changed often and a system with fresh fluid
will most likely not contain water.

Because of this, racers should be concerned with the dry boiling point.
Racing fluid exceeds DOT 3, 4, and 5.1 dry boiling point specifications.
Never use silicone based fluids in racing brake systems.

Using racing brake fluid will increase performance of the braking system.
Never reuse fluid. º Never mix types or brands of brake fluid.

Use smaller fluid containers that can be used quicker.

If fluid remains in container be sure to tightly seal and do not store for
long periods of time.

Purge system (complete drain) and replace fluid often.

Immediately replace master cylinder reservoir cap following any maintenance.