Brake fluid absorbs heat and dissipates it throughout the braking system, but can also absorb water and heat, causing damage. It should be changed periodically to remove accumulated water and to withstand high temperatures. Air trapped within a brake system can lead to brake failure. Bleeding the brake system removes air and fluid may need to be topped up.
Brake fluid is a type of hydraulic fluid used in vehicle brake systems. Typically a clear fluid, brake fluid has the ability to absorb heat and dissipate it throughout the braking system. A typical disadvantage of brake fluid is its ability to absorb water and heat: water in brake fluid can damage brake calipers and cause rust in brake lines. Although not routinely practiced by most vehicle owners, brake fluid should be changed periodically to remove accumulated water from the system. When the brake fluid begins to turn a dark color, this is an indication that it needs to be serviced and replaced.
The main reason why brake fluid should be changed is excessive heat. The fluid found in most passenger vehicles has a very low tolerance for heat. It is capable of withstanding normal braking heat and the occasional extreme braking. In racing and severe service applications, a high performance fluid should be used. The special racing fluid is formulated to withstand the extreme heat often generated by continuous deceleration from ultra-high speeds.
The fluid found in most braking systems is organic hydraulic fluid. As such, it burns on exposure to intense heat. When burned, the liquid turns from clear to brown or black and has a noticeable burning odor. In certain high performance applications, the fluid used in brake systems is synthetic.
This synthetic brake fluid is better able to withstand high temperatures without burning or discoloring. Even synthetic fluids are subject to water absorption and must be changed periodically. Each fluid manufacturer has recommendations for fluid change intervals.
As with any hydraulic system, air is the enemy in a brake system: air trapped within a brake system can lead to a soft-feeling brake pedal and failure of the brake to engage when the pedal is depressed. All hydraulic brake systems have a brake bleeder valve located on each brake cylinder. By pumping the brake pedal and holding it to the floor, the bleeder valve can be opened with a wrench and trapped air can escape. The bleed valve closes immediately and the process repeats until only fluid is coming out of the bleed valve and no air is present. Often the brake master cylinder will require topping up the brake fluid while bleeding the brake system.
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