Symptoms of a Bad Brake Booster

by Herb Kirchhoff
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Because of the prevalence of disc brakes, power-boosted braking systems now come standard on nearly all new automobiles and recent-model used vehicles. Disc brakes are not self-energizing like drum brakes, so they require a power brake booster to keep pedal effort reasonable for all drivers. But like any human-made device, power brake boosters can go bad.

Booster Failure

The primary symptom of power brake booster failure is a high, hard pedal that requires greater than normal pedal pressure to stop the car. You may also notice that it takes a much longer distance than normal to stop your car The high, hard pedal and longer stopping distance mean you are not getting power boost.

Proper Function

A properly functioning power brake system should provide power assist on first application of the brakes every time, with normal pedal travel, pedal pressure and stopping distance. However, if your brake pedal is low, spongy, requires pumping or fails under steady pressure, the brake booster likely is not at fault. These are indications of other serious problems in your braking system that require prompt attention.

Testing Booster

Most power brake systems in autos are vacuum-based, using the difference between engine intake vacuum and atmospheric pressure as their power source. To test your brake booster, pump the brakes several times with the engine off to deplete stored vacuum. Turn on the engine while pressing lightly on the brake pedal. You should feel the pedal fall away a bit and then become firm, but not hard. If you feel nothing at the pedal when the engine starts, your brake booster is not functioning.

Another Test

Booster problems may indicate failure of the booster unit itself or problems in the vacuum system that powers the booster. A vacuum system problem may leave you with only partial boost, according to To test for vacuum problems, start the engine and run it to medium speed, then turn off ignition and take your foot off the gas. Wait about a minute and a half and then apply your brakes. You should feel the vacuum boost on at least two brake applications. If you do not, you may have a vacuum system leak or bad vacuum check valve.

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