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Low Brake Pedal

The brake pedal should be checked for proper adjustment any time a low brake pedal is being diagnosed. The brake pedal is typically adjusted with the master cylinder pushrod. The pushrod is usually (almost always) made of two parts and is lengthened or shortened by adjusting a nut located on the rod. A misadjusted brake pedal is often the cause of a low brake pedal.

Another culprit is likely to be contaminated, overheated, or aerated brake fluid. Brake fluid is hygroscopic meaning it attracts moisture. If a container of brake fluid is left open it will attract moisture and quickly become contaminated.

The same is true with heat. Heat from the braking system can cause the brake fluid to boil. This causes it to vaporize leaving pockets of vapor in the brake lines. The whole idea of using a fluid like brake fluid is the fact that it is not compressible. On the other hand a vapor like the one caused by overheating the brake fluid is. This vapor in the brake system will definitely result in a low brake pedal.

Brake fluid is rated by the Department of Transportation, the DOT. They rate the different brake fluids by assigning them a number like DOT3, DOT4, and DOT5. The higher the DOT rating the higher boiling point (the more heat it takes to boil). DOT5 brake fluid is silicon based where the other two are glycol based. Never mix the two as this will definitely cause a low brake pedal or at least a loss of performance. Moisture in a brake system will lower the boiling point indicated in the chart below.