[HP Autosport]

Carroll Smith, a respected and winning professional racer, team manager and race car engineer. If you have not read some of his books(e.g., Tune To Win and Engineer To Win), read them.

Here is what he had to say with regards to brake fluid:

Carroll Smith's Notes on Brake Fluid

Brake fluid is possibly the single most neglected component of the automobile. Most high performance drivers check their tire pressures and change their engine oil at frequent intervals. Virtually no one (including me) ever changes the brake fluid in their street car - or even bleeds the brakes. WRONG!

The function of brake fluid is to provide an incompressible medium to transmit the driver’s foot pressure on the brake pedal through the master cylinder(s) to the calipers in order to clamp the friction material against the discs. The foot pressure is multiplied by the mechanical pedal ratio and the hydraulic ratio of the master cylinders, booster (if used) and caliper piston(s).

This is a simple concept. When fresh, all brake fluids are virtually incompressible and the system works as well as its mechanical and hydraulic design allows. There are, however significant problems. Overheated brake fluid can (and will) boil in the caliper. Boiling produces gas bubbles within any boiling fluid. Gas is compressible so boiling brake fluid leads to a “soft” brake pedal with long travel. In extreme cases overheated brake fluid necessitates “pumping the brake pedal” in order to get a pedal at all.

There are five possible solutions to the boiling fluid problem:

1. Don’t use the brakes so hard
2. Provide (copious) cooling air to the brakes
3. Install Titanium or Ceramic caliper pistons. Alternately install 2-piece pistons made from aluminum with a noise piece of Titanium or Ceramic. (Note that aluminum pistons are used as opposed to stainless steel to match the expansion rate of the piston to that of the caliper body to ensure the piston seal condition and preload is optimal through out the temperature range.)
4. Install Titanium pad backing plate if they are available for your caliper.
5. Change to a brake fluid with a higher boiling point.

The first is unacceptable. The second is expensive. The third is really expensive in its various forms. The fourth is relatively cheap. The fifth is easy and relatively cheap.

For most of us easing up on the brake usage is not an option and increasing cooling air to the system is filed under too difficult so we upgrade the fluid. Some of the reputable racing fluids include:

· AP 550
· AP 600
· ATE Super Blue Racing
· ATE TYP 200
· ELF HTX 115
· Motul 550
· Motul 600
· Neo 610
· Performance Friction Z rated

Finally, Castrol SRF is a racing brake fluid that is in a class by itself with patented chemistry and is, in my opinion, the best racing brake fluid on the market today.

This leads to a discussion of boiling points. Brake fluids are classified by both “dry boiling point” and “wet boiling point”. They are also classified by US Department of Transportation (DOT) rating, DOT 3, DOT 4, DOT 5, and DOT 5.1.

As we would expect the dry boiling point is just that - the temperature at which a given brake fluid boil when it is fresh out of the can. This is the rating by which most high performance drivers and all racers select their brake fluid – from the standard racing 550 degrees Fahrenheit to the 600+ degrees Fahrenheit offered by the extreme use fluids. As a point of interest, even though they may have the same DOT rating, racing fluids are less compressible than street fluids, especially after they have been overheated.

For high performance street car use, the wet boiling point is at least as important as the dry. DOT 3 DOT 4, and DOT 5.1 brake fluids are ether based and, as such they are hygroscopic in nature - i.e. they adsorb water at every opportunity. Since water boils at 212 degrees Fahrenheit (100 degrees Celsius) the adsorbed water dramatically lowers the boiling point of the brake fluid. A minute amount of water suspended in the fluid decreases the boiling point as much as 1/3. Damn!

The fluid in the system absorbs water through the breathers, through the caliper piston seals and by magic. Not only does this reduce the boiling point, the entrained water leads to corrosion of both ferrous and Aluminum internal parts. Double Damn!! So buy your brake fluid in small containers and don’t save the leftovers.

I use Ford C6AZ-19542 which was developed in the early 1960’s to cure the problem caused by Lincoln Continental drivers boiling the fluid by habitually resting their left feet on the brake pedal. It is inexpensive and it works just fine.

But upgrading the fluid is not the whole answer. Unfortunately the hygroscopic nature of the ether based fluids means that they should be completely replaced at scheduled time based intervals (annually would be good) and that the system should be bled to replace the fluid in the calipers every time that it is overheated to the point of generating a soft pedal. Yes, the pedal will come back as soon as the fluid cools somewhat - but the boiling point is now reduced and the pedal will go mushy at a lower temperature the next time. Triple Damn!!!

Fortunately, changing to a 550 degree Fahrenheit fluid and replacing it annually will solve the problem for all but the most heavy footed among us.

Ah ha, you say - but what about the much touted Silicone based brake fluids? They are non hygroscopic and should take care of the reduced boiling point and corrosion problems. True! That’s the good news. That is why they are specified by the U.S. Military. Unfortunately the silicone based fluids are compressible themselves so they produce a soft pedal all by themselves. For the person who doesn’t care about a spongy pedal or precise modulation silicone fluids may well be the answer - but not for anyone reading this. In fact, low compressibility is a desired characteristic in a high performance brake system – lower compressibility results in more linear force output for driver input and improved driver feedback.

So, you say, “I’ll just run my hot rod down to my dealership or mechanic and ask them to replace the fluid with AP 550.” Two problems here. First of all they won’t have it and won’t know where to get it. Second and more serious (after all you could supply it yourself) all dealerships and most independent mechanics use pressure bleeders. These devices certainly speed up the process of brake bleeding, but by forcing the fluid through tiny orifices using pressurized air, any air left in the partially filled system will be forced into the fluid solution. Quadruple Damn!!!!

If you are either racing or doing lapping days, the brake fluid should be replaced both before and after each event. A cool off lap before each stop will go a long way toward preventing fluid boil due to heat soaking after the airflow stops when the car does.