Chapter 10

All About Brake Fluid

Brake systems for cars, trucks, motorcycles and airplanes all use "DOT" fluid. Even in the smallest third-world outpost DOT is the term used by car and truck mechanics. DOT is shorthand for the U.S. Department of Transportation, which long ago determined a specification for the performance of brake fluid. The original DOT specification was replaced by DOT 2 which, when disc brakes became common, was replaced by DOT 3. Today's high performance cars with antilock brakes need a better fluid still, DOT 4. When certain racing motorcycles needed a fluid that could withstand temperatures hot enough to make discs glow red, the U.S. Department of Transportation developed a standard for DOT 5.

The problem is that while DOTs 2, 3, and 4 are all compatible with each other, none are compatible with DOT 5. If you add DOT 5 to a system that already contains 2, 3 or 4, the resulting mixture will dissolve the rubber 0-rings common to all hydraulic systems. Within a couple of hours the brake system will leak like a sieve, and the fix requires a complete system overhaul with all new rubber parts. Not cheap, or easy. The moral of this story so far: Don't ever mix DOT 5 (known to most as "the blue stuff") with any other DOT fluids (which are all the color of motor oil).

We wish that were the end of the story. But progress marches on and eventually a higher-temperature-rated version of DOT 4 was developed. While we think someone should have named it DOT 4.1 or DOT 6, it is instead labeled DOT 5.1. It is this super- fluid, DOT 5.1, which was installed in your Formula brake at the factory. Unfortunately 5.1 is extremely hard to find. When you ask for it, people will typically try to sell you DOT 5. If you are simply topping off or bleeding fluid we recommend DOT 4, which is totally compatible with DOT 5.1. If you are refilling the entire system, call us and we will sell you DOT 5.1.

Wet vs. Dry Boiling Temperatures

DOT brake fluid is "hydrophilic". This means the fluid will absorb water from the air. Rumor has it that you can fill a shot glass with DOT fluid and after an hour or so the glass will overflow. The DOT brake fluid absorbing moisture from the surrounding air causes the volume increase. This is why every container of DOT fluid requests that you keep it closed when not in use. As the fluid absorbs water its vaporization temperature (or boiling point) is reduced. DOT 4 becomes no better than DOT 3 and eventually degrades in performance to the level of DOT 2. Amazingly enough, a small amount of moisture will be absorbed right through rubber seals and nylon hydraulic lines (virtually any material except metal or glass). This is why car manufacturers recommend brake fluid replacement every two to three years.

Because fluid that exists in a hydraulic system will have a lower boiling point than when new, the Department of Transportation specifies two minimum boiling points for each brake fluid, known as 'dry' and 'wet.' The distinction 'dry' is given to fluid uncontaminated by water, while 'wet' is the expected boiling point for a fluid with approximately four percent water content, the average amount the DOT found when randomly testing truck brakes. 'Racing' brake fluid found in a high- performance auto parts store will boast a high dry boiling point. Less expensive fluid might have a higher wet boiling point. Which is best? For racers who replace their brake fluid following every event, dry boiling point is all important. Unless you wish to replace your brake fluid after every ride, ignore the dry temperature and choose a fluid based on its wet boiling point. The minimum wet boiling point that qualifies a fluid for DOT 4 status is 311 degrees Fahrenheit. The synthetic version of Valvoline's DOT 4 has a wet boiling point of 333 degrees, making it the best fluid you are likely to find in your hometown.

What's so good about 5.1?

DOT 5.1 has a wet boiling point of 365 degrees. Better yet, DOT 5.1 is half as viscous as DOT 4. This means a system filled with DOT 5.1 provides quicker lever response, better modulation and faster pad retraction When servicing, the lower viscosity facilitates bleeding and pad adjustment. DOT 5.1 also experiences less volume change as a result of temperature shift than DOT 4, allowing more consistent braking through a range of temperature. It may require a little more effort to purchase DOT 5.1, but we feel it is well worth it. In every case make sure any fluid used is labeled 'DOT 3', 'DOT 4', 'DOT 5.1' or 'Compatible with DOT 4'. You can obtain Motul 5.1 from Santana or through your Santana dealer.

Warning: Other bicycle disc brake systems use common mineral oil or a proprietary 'miracle' fluid (snake oil?). A well-meaning bike shop mechanic might not realize the can of "Super Fluid" he has on his shelf is totally inappropriate for your automotive-quality brake. While bicycle disc makers often use a less caustic (and cheaper) fluid, these fluids will have higher compressibility, greater expansion and contraction, and a lower boiling point. In short, when performance matters, there is ample reason to use DOT fluids (except DOT 5) in spite of the additional care required.

[Home] [Tech Tips] [Formula Disk Brake Table of Contents] [Next Chapter]

Problems or Questions about this site? Email the WebGuy