Brake Fluid 101

[Ed]

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.

SILICONE BASED FLUID

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.

POLY GLYCOL ETHER BASED FLUIDS

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.

FLUID SPECIFICATIONS 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 fluids).

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.

WET VS. DRY BOILING POINT

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

WET BOILING POINT

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

DRY BOILING POINT

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

MOISTURE IN THE BRAKE SYSTEM

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.

THINGS TO REMEMBER

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.

[Daniel]

This post seems to have hit at a perfect time. I visited with a friend yesterday that is a nissian tech.He had just returned from brake school. Per his instructor the silicone is for race cars only. The high temp makes it good for that purpose. The fact it doesn't mix with water does not mean there will be no water in the brake system. It should be changed at a minimum once a year. It will destroy the metal in the brake system with pits and scars and cause premature failure of rubber parts. It has no reason to be in a street vehicle. I guess once again.If you thought you out thought Datsun,you better think again!!

[Tourniqet]

Originally posted by Ed

Purge system (complete drain) and replace fluid often.

when purging how would you get all the fluid out of the lines? I would think there would be fluid left over unless it was flushed somehow...

[ChrisA]

Purging/flushing is kind of relative. You keep draining and adding fluid until you feel that you've gotten most or all of the old fluid out. I have a professional bleeder that will pull the old fluid out while a tank keeps adding to the master cylinder at the same time. Usually you keep going till the fluid coming out is the same color as the fluid going in. Its always worked for me. Might be a tad wasteful on brake fluid but your brakes and your safety are not something to scrimp on.

Come on members, give us some more thoughts on this one.

Chris

[Gav240z]

The Haynes manual says to flush with Methylated Spirits. Just came across this thread trying to see if the brake fluid im using is ok. Seems to be fine as it's DOT3-4.