Brake Fluid Basics – Understanding the Fine Points Can Help You Select the Best Fluid for Your Needs

Racing is all about speed. Well, it is for a little while anyway. Then, because somebody decided to put a corner right in the middle of a perfectly good straightaway, racing is all about slowing down.

However, there is a great deal more to braking than just stomping on the middle pedal. There are complex mechanical and hydraulic systems at work, with spinning metal mating with exotic pad compounds and massive amounts of heat created from slowing a car from ludicrous straight-line speeds to more reasonable cornering speeds. That lower, more reasonable speed is the correct speed that keeps the front of your car from impacting the tire wall. Because repairing tire wall damage is no fun, I think we can all agree that slowing down in racing is every bit as important as speeding up.

One component of a braking system that is probably the easiest and least expensive to improve and maintain is the brake fluid. In a perfect system, we wouldn’t need brake fluid, instead there would be a steel rod at the bottom of the driver’s foot and when he pushed down on that rod, the steel rod would then press the brake pad directly into the rotor. In that system there would be no mechanical loss. However, that system wouldn’t work very well on a car that needs to turn or has any type of suspension. Obviously, the steel rod at the bottom of the foot idea is out since some genius slapped a corner in the middle of a great straightaway, therefore, our brake system needs hydraulics to allow for suspension and steering articulation.

Now that we know we need hydraulics to make the brakes work, we need to decide what would be the best type of fluid to use. How about water? Water doesn’t compress and it’s cheap. The problem with water is that it changes form from a liquid to a gas at a measly 212 degrees Fahrenheit. Water in a liquid state is uncompressible, like our steel rod design. However, water in a gas state is very compressible.

Once the water boils and goes from a liquid to a gas, our brake pedal will go to the floor. It will do this because instead of the brake pedal motion pushing the water through the brake lines, ultimately pushing the caliper piston, the motion of the brake pedal will just compress the gas. Compression is not what we are looking for. How quickly will a brake system heat up to convert water from liquid state to gas state? I would think you could probably boil the fluid from just backing your car out of the race trailer. Water is a great coolant, but a terrible brake fluid.

If water is out of the question, then what type of hydraulic fluid should we use? To answer this question, I spoke with Michael Wachholz, who is known as the “Brake Fluid Guru.” Michael is the founder and owner of Torque Inc., based in Sonoma, Calif., which produces RT700 Racing Brake Fluid, the only racing brake fluid developed and produced in the USA. If you think you know a lot about the subject of brake fluid, have a conversation with Michael Wachholz and you will quickly realize you don’t know jack.

First, for brake fluid to actually be “brake fluid” it has to meet criteria set forth by Federal Motor Vehicle Safety Standard 116. FMVSS 116 stipulates that brake fluid has to have high dry and wet boiling points, be compatible with components made from a variety of rubbers, provide lubrication and assist with swelling brake system seals, maintain a pH of 7 (neutral) to 11.5 (alkaline), be resistant to oxidation, meet specific viscosity characteristics at very low and high temperatures, provide corrosion resistance and have minimal changes to its performance over time and use. That is a pretty tall order for a hydraulic fluid to meet. So again, simply putting water in your brake lines won’t do the trick. It wouldn’t even pass the first criteria of dry or wet boiling points.

So what does this wet and dry boiling point stuff mean? Isn’t brake fluid always “wet?” Yes, brake fluid is wet, and you could say it actually gets “wetter” since brake fluid is hygroscopic, which means it absorbs moisture. The wet and dry boiling-point measurements differentiate between a fluid that is fresh out of a sealed bottle and a fluid that has been exposed to the atmosphere and has absorbed moisture over time.

In an automotive brake system, moisture is a fact of life. There is no brake system that is sealed perfectly so moisture cannot enter the system. Wachholz said moisture uptake — brake fluid industry nerd speak for absorbed moisture in the brake fluid — is highly dependent on the environment. It is difficult to suggest a specific time period for a DOT 4 performance-oriented fluid to have absorbed enough moisture to be reduced to its wet boiling point. This is all related to whether you race in Florida where it’s very humid, or whether you race in southern Arizona where there is not as much water in the atmosphere. However, the rule of thumb from the industry is DOT 4 fluids have a 24 month life cycle. Wachholz recommends a performance minded driver would be wise to flush and refill the entire system every 12 months.

Because fluid absorbs moisture in the car or sitting in a bottle, I started using a label maker to mark when my bottles were purchased and when we opened them. I began this procedure after standing in my racing trailer staring at three opened bottles of brake fluid wondering, “Which one of these has been sitting in here for five years?”

Something racers might not consider is that unlike almost every other fluid in an automobile, brake fluid doesn’t recirculate. What that means is that the brake fluid in your calipers now is the same fluid in your calipers that has been heated and cooled hundreds of times over. Professional teams will flush and/or bleed their brakes after each session as a preventive measure to have fresh fluid in the calipers at all times. Amateur teams, HPDE drivers and car guys, typically bleed the brake system between events or when they feel the pedal beginning to go soft. This loss of pedal feel or pedal consistency is a function of brake fluids with decreased chemical and high-temperature stability.

“The factors of high-performance or racing brake fluid resulting in reduced or almost nonexistent compressibility and an absolute and consistent pedal feel are more important than a high dry boiling point number,” Wachholz said.

Wachholz uses a lot of big words when he speaks about brake fluid, I’ll try to interpret what he is trying to say: A good brake fluid shouldn’t begin to fade during a long session. Period.

During a 25 Hours of Thunderhill race, my team ran a beta version of Torque RT700. We changed Carbotech brake pads three times but never bled the brakes once. The fluid held up solid with no pedal fade for 25 hours of continuous racing. I still don’t know what Wachholz is talking about when he says “nonexistent compressibility,” but I do know the pedal was stiff, and I was convinced the fluid was working.

So the next time you go to add some brake fluid in your car, you should know how old your fluid bottle is. The older it is, the more moisture it has absorbed. Consider how hot your brake system will get and purchase a fluid with a dry boiling point above that temperature. Also remember, when you are flying into a turn and you are stomping on the whoa pedal, the one thing that is keeping the front of your car from colliding with the tire wall is some relatively inexpensive brake fluid that is easy to replace.

DOT Numbers by the Numbers

There is always a bit of confusion about DOT brake fluid classifications and which fluids can mix with which. The standards for brake fluid DOT classifications come from Federal Motor Vehicle Safety Standard 116 and from Society of Automotive Engineering papers J1703 and J1704. Unless you are an engineer, you probably don’t want to wade through these papers. Here is a basic breakdown of the fluid standards and which ones you can’t mix.

Data From Brake Fluid Manufacturers

Some people just want the quick answer. Which fluid is the best for racing? To answer that question, most people jump to the next question. Which one has the highest dry boiling point number? Even though there is a lot more to brake fluid performance than just the highest dry or wet boiling point, here is a list from the highest to the lowest based on brake fluid manufacturer data.

What about Silicone DOT 5?

Silicone fluids are not for racing. At first glance, the boiling point numbers look good, however silicone refuses to absorb moisture. That is a good thing right? Wrong. If the brake fluid does not absorb moisture and distribute it evenly within the fluid, you will end up with a silicone-water-silicone situation in your brake line. In that scenario the water will boil, allowing compression of gas and a pedal that goes to the floor. Additionally, silicone brake fluid is known for having a high percentage of dissolved air — as much as 19 percent — in the solution, which again is something you don’t want in a racing brake fluid. Silicone is popular with show cars and custom choppers because the fluid will not harm paint if it is spilled. DOT 3, 4, and 5.1 will definitely put a hole in your paint job. Bottom line: Save the silicone fluids for the trailer queens.

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