Of critical importance in determining a fluids ability to handle high temperature applications is the Dry Boiling Point and compressibility.
The Dry Boiling Point is the temperature at which a brake fluid will boil in its virgin non-contaminated state. The highest temperature Dry Boiling Point available in a DOT 3 fluid is 572° F.
The Wet Boiling Point is the temperature a brake fluid will boil after it has been fully saturated with moisture. The DOT 3 requirement for wet boiling point is a minimum temperature of 284° F.
There are many ways for moisture to enter your brake system. Condensation from regular use, washing the vehicle and humidity are the most common, with little hope of prevention. Glycol based DOT 3 & 4 fluids are hygroscopic; they absorb brake system moisture, and over time the boiling point is gradually reduced. Here is a scary statistic, a new car at 12 months old will have a 2% water content if never driven, at 18 months it will be 3% or higher depending on atmospheric conditions. It is not unusual to have 8-12% water content in the average automobile on the street.
You should not use DOT 5 fluid in any racing application for the following reasons. DOT 5 fluid is not hygroscopic, so as moisture enters the system, it is not absorbed by the fluid, and results in beads of moisture moving through the brake line, collecting in the calipers. It is not uncommon to have caliper temperatures exceed 200° F, and at 212° F, this collected moisture will boil causing vapor lock and system failure. Additionally, DOT 5 fluid is highly compressible due to aeration and foaming under normal braking conditions, providing a spongy brake feel. DOT 5 fluid is best suited for show car applications where its anti-corrosion and paint friendly characteristics are important.
Whenever you add fresh fluid to your existing system (never mix fluids of different DOT classifications), it immediately becomes contaminated, lowering the boiling point of the new fluid. For maximum performance, start with the highest Dry Boiling Point available, flush the system completely, and flush it regularly, especially after severe temperatures have been experienced.
If you happen to mix DOT 3/4 and DOT 5 you will end up with a coagulated gel that will seriously compromise your braking performance. Dot 5.1 is a Glycol based fluid like DOT 3 or 4 and should not be confused with DOT 5 which is silicone based, many new European cars specify DOT 5 but in Europe they don't recognize the silicone based (DOT 5) fluid so they really mean DOT 5.1, ensure you don't confuse the two. DOT 5.1 was developed for ABS braking systems and is of a thinner viscosity to allow shorter cycle timing, it's boiling point is similar to DOT 3 or 4.
Much of this info I cut and pasted from the Wilwood brakes site and I have had similar expierience with GT3 racing cars when I tried DOT 5, it was a waste of time and money.
[ April 09, 2002: Message edited by: 4 Jaw Chuck ]</p>
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