Whether driving to work or enjoying a track day, the most important mechanical system in your car or powersports vehicle may be your brakes. Within the braking system, one of the most often overlooked performance parts is the brake fluid.

The purpose of brake fluid is to convert brake pedal pressure into stopping power while remaining stable in a variety of conditions. When the brake calipers (and the brake fluid reaching them) get hot, the fluid will often boil. The process of boiling produces gas which is more compressible than the brake fluid leading to a soft, spongy brake pedal feel and a longer travel time when applying the brakes. Over time, hygroscopic brake fluid begins absorbing water from the atmosphere.

Brake fluid containing water boils with less heat. Because brake rotors and brake calipers generate a high amount of heat, brake fluid must have a high heat tolerance — otherwise, the fluid would simply vaporize inside your brake lines and create a dangerous situation as your brake system will not function properly.

The US Department of Transportation, DOT, established specifications defining a number of properties to which brake fluid must adhere. Those specifications relate to the boiling point of the fluid (both dry and wet), how viscous (flowable) the fluid is, and the stability of the fluid at high temperatures, among other properties. Dry boiling point refers to when the brake fluid is new (0% water), while wet boiling point refers to when the water content in the brake fluid reaches 3.7%.

 

The most common brake fluid is DOT 3 and it is primarily glycol ether. DOT 4 fluids are also glycol ether-based, but have borate esters added to increase the boiling points. DOT 5 fluid was manufactured using silicone which does not absorb water.

 

Brake Fluid Classifications

Dry Boiling Point Wet Boiling Point
DOT 3 205o C / 401o F 140o C / 284o F
DOT 4 230o C / 446o F 155o C / 311o F
DOT 5 260o C / 500o F 180o C / 356o F
DOT 5.1 270o C / 518o F 190o C / 374o F

 

Source: Federal Motor Vehicle Safety Standard No. 116, Motor Vehicle Brake Fluids (49 CFR 571.116)

Maintaining brake fluid via regularly scheduled service replacement intervals is important or the brake fluid performance will decline as the fluid ages. Brake bleeding intervals are heavily dependent on vehicle use and the type of brake fluid being used.

 

For a street car where a balance of performance and affordability may be important, you might consider bleeding your brakes and replacing the fluid once per year.

 

For a combination street and track car, the brake priorities change and so a fluid with low compressibility becomes important to maintain a consistent pedal feel. A high dry boiling point is also important to perform under sustained track sessions. For track use, it is recommended that you consider bleeding the brake calipers after each weekend and replacing the brake fluid.

For a race car, you want a brake fluid with high lubricity, dry boiling point (above 600o F) and low compressibility for maximum stopping power and resistance to brake fade.

The experts at VP Racing Fuels offer a 622 Racing Brake Fluid™ formulated for the demands of automobile and powersports racing, and can also be used on the street in extreme performance vehicles. It is a synthetic brake fluid that maintains stability under high temperatures, resists vapor lock and fade, and delivers the highest dry boiling point of any DOT 4 fluid (622 o F) on the market.