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What is Brake Fluid?

By Lori Kilchermann
Updated May 23, 2024
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Brake fluid is a type of hydraulic fluid used in vehicle brake systems. Usually a clear liquid, brake fluid has the ability to absorb heat and dissipate it throughout the braking system. A typical downside of brake fluid is its ability to absorb water as well as heat—water in brake fluid can cause damage to brake calipers as well as generate rust in brake lines. While not routinely practiced by most vehicle owners, brake fluid should be changed periodically to rid the system of any accrued water. When brake fluid begins to transform to a dark color, this is an indication that it should be serviced and changed.

The single biggest reason that brake fluid should be changed is excessive heat. The fluid found in most passenger vehicles has a very low tolerance to heat. It is able to withstand normal braking heat and occasional extreme braking. In racing and severe duty applications, a high-performance fluid should be used. Special racing fluid is formulated to withstand the extreme heat often generated by the continued deceleration from ultra-high speeds.

The fluid found in most braking systems is organic hydraulic fluid. As such, it is burned by exposure to intense heat. When burned, the fluid turns from clear to brown or black and has a noticeable burned odor. In certain high-performance applications, the fluid used in brake systems is synthetic.

This synthetic brake fluid is better able to withstand high heat without becoming burned and discolored. Even the synthetic fluids are subject to water absorption and should be changed periodically. Each fluid manufacturer has recommendations for fluid change intervals.

As in any hydraulic system, air is the enemy in a brake system—air trapped within a brake system can lead to a mushy-feeling brake pedal and failure of the brake to engage when the pedal is depressed. All hydraulic braking systems have a brake bleeder valve positioned at each brake cylinder. By pumping the brake pedal and holding it to the floor, the bleeder valve can be cracked open with a wrench and the trapped air can escape. The bleeder valve is immediately closed and the process is repeated until only fluid escapes the bleeder valve and no air is present. Often, the brake master cylinder will require refilling of brake fluid while bleeding the brake system.

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