How to Diagnose a Pulsating Brake Pedal

by eHow Cars Editor

A pulsating brake pedal may indicate brake parts worn beyond their limits or loose components. Excessive lateral run-out or thickness variation also may cause this condition. Diagnose a pulsating brake pedal by implementing the following steps.

Determine if the pedal feels firm or mushy. A soft, mushy or spongy feeling could result from leaks in the brake lines. Examine the pedal to see if it slowly sinks which may indicate a worn-out master cylinder. Look for worn or loose brake parts.

Look for excessive lateral run-out. This refers to a rotor's wobbly movement. Rotor run-out should not exceed .002 and .005 inches. Brakes may produce pulsation, noise or vibration when the run-out exceeds this range. Excessive run-out can push the pads too far from the rotor.

Inspect loose components, such as wheel bearings or calipers that can cause rotor thickness changes. Lack of parallelism or variation in rotor thickness can produce the same factors that cause run-out. Look for loose wheel bearings, lateral rotor run-out, uneven torque or a dragging caliper that causes the friction material to contact the rotor.

Pinpoint parallelism with a micrometer that measures rotor thickness for at least four equally spaced points about an inch from the edge.

Examine lug nut torque. A lug nut with five pounds more pressure on it can cause a rotor to warp. When tightening the lug nuts, follow a two-step process in a star pattern and use a torque wrench, a tightening device that yields equal pressure. Tighten the lug nuts to half the necessary torque, and then use the wrench to secure the nuts to the final torque specification.