Braking is really about heat management. When you apply your brakes you’re converting mechanical energy into thermal energy. Remember high school physics? You can’t destroy energy. You can though convert it.
Mechanical energy gives you speed. Braking is an energy conversion process. As you brake into the corner you’re converting all that mechanical energy into thermal energy. That thermal energy is then dispersed into the atmosphere.
Most brake rotors even have vanes between the two friction surfaces in an effort to get all that thermal energy into the atmosphere and away from the car.
You may even have ducts that direct cool air at the center of the brake rotor, which then blows the super hot air into the atmosphere. You can only do so much though. Your brakes are still going to get hot. Very hot.
The friction between your brake pad and your rotor generates some serious heat. I see 1200-degrees on a normal weekend at Sebring. That means my brake calipers are getting really really hot. If my brake calipers get that hot then the brake fluid inside my calipers is getting very hot as well. While the fluid will never get as hot as the brake rotor it’s entirely possible that it can reach 500-degrees.
Brake fluid requires a very high boiling point to avoid vaporizing, or becoming steam, in the brake lines. This is a problem because steam is compressible. When you have steam in the brake lines you don’t have brakes. When you push on the pedal all that happens is that you compress the steam. The brake pads never move and the car doesn’t stop.
Every weekend a number of people have the brake pedal drop to the floor during a race. Then after five minutes in the pits the pedal acts normal. What happened was that the steam in the brake lines returned to being a liquid. In other words everything returned to normal. If they had put new brake fluid in the brake system before the event there might never have been a problem.
The big advantage of new brake fluid is that it contains very little moisture. Remember, water always boils at 212-degrees. Water enters your braking system because brake fluid actually sucks the moisture out of the air.
The technical term is that brake fluid is hydroscopic. Keep in mind that brake fluid will attract moisture whether it’s in your braking system of sitting on an open can in your shelf. That’s why you should always start brake bleeding with a new can of fluid.
People always refer to "dry" and "wet" boiling points. The wet boiling point refers to the fluid's boiling point after absorbing a certain amount of moisture. Since water boils at 212 degrees Fahrenheit (100 degrees Celsius) the absorbed water dramatically lowers the boiling point of the brake fluid. A minute amount of water suspended in the brake fluid decreases the boiling point as much as thirty per cent.
DOT 3 brake fluids must have a minimum dry boiling point (measured with 0% water) of 4010F and a minimum wet boiling point (measured with 3.7% water) of 2840F.
DOT 4 brake fluids must have a minimum dry boiling point of 4460F and a minimum wet boiling point of 3110F.
DOT 5.0 brake fluids are the wild card. They’re a synthetic brake fluid and should never ever be mixed with any other type of brake fluid. This synthetic fluid is never used in a race car so I’m not going to spend a lot of time discussion it except to point out that you should never mix it with the other fluids discussed here.
DOT 5.1 brake fluids are DOT 4-type fluids which meet DOT 5 performance requirements. Because of this, they typically can be mixed with DOT 3 or DOT 4 fluids without concern.
In some circles, 5.1 fluids are even referred to as DOT 4 Plus or Super DOT 4 fluids because they’re more similar to a conventional DOT 4 fluid by chemistry than they are to a conventional DOT 5 fluid.
The big advantage of the DOT 5.1 brake fluid is that it contains all the water-absorbing characteristics of the DOT 3 and DOT 4 fluids while simultaneously providing for very high boiling points and relatively stable viscosity over a wide range of temperatures. The best of all worlds.
One thing you should notice in the chart is that a high wet boiling point doesn’t mean you have a high dry boiling point. Castrol SRF has the highest wet boiling point at 518-degrees. AP 600 on the other hand has the best dry boiling point ay 608-degrees. AP 600 though is considerably below the Castrol SRF when it comes to moisture-laden fluid. Read the specifications very carefully.
I’ve known people who bleed the brakes every morning of race weekends. That’s not a bad thing. No harm can come from too much brake bleeding. The harm comes from not bleeding your brakes often enough. It also comes from using brake fluid cans that you opened several months ago.
I firmly believe that how often you change your brake fluid is far more important than the particular brand you use. If you always begin with a newly opened can and you change the fluid out before each event you should be fine.
Brand Name | Dot Rating | Wet Boiling Point | Dry Boiling Point | Compatible With Conventional Brake Fluids |
Castrol SRF | 4 | 5180 | 5900 | Yes |
Motul RBF 600 | 4 | 4210 | 5940 | Yes |
ATE Super Blue | 4 | 3880 | 5360 | Yes |
ATE Type 200 | 4 | 3880 | 5360 | Yes |
AP PRF 600 | 4 | 3990 | 6170 | No |
AP 600 | n/a | 3990 | 5940 | No |
AP 551 | 3 | 3040 | 5160 | Yes |
AP DOT 5.1 | 5.1 | 3690 | 5160 | No |
Castrol LMA | 4 | 3110 | 4460 | Yes |
*All Temperatures are in Fahrenheit
