Test an 88 GMC Brake Booster

The brake power booster uses vacuum generated in the engine intake manifold to amplify the force the driver exerts on the brake pedal, making the driver's job easier when slowing or stopping the vehicle. This is especially important on large vehicles like those made by GMC. Problems with the power booster can create a dangerous situation because the driver may unexpectedly need much greater effort when braking. If the braking seems to be more difficult than normal it is time to test your brake booster. Testing the booster is an easy task that every driver can do to assure herself that it is functioning properly

Procedures:
1. Park your 88 GMC and turn the engine off. Pump the brake pedal slowly five or six times to relieve any residual vacuum in the booster. Push the brake pedal down and hold it down with light but steady force. The brake pedal should feel harder and higher than normal. Start the engine. The brake pedal should immediately drop 1/4-inch or so and it should suddenly feel normal. If your booster is not functioning properly the pedal will stay in a high position and will feel hard.
2. Let the engine idle for a couple of minutes. Turn the engine off and slowly pump the brake pedal, waiting a couple of seconds between each pump. The pedal should go down to a normal position on the first pump, and then stop a little bit higher with each following pump.
3. Start the engine again and let it idle for a couple of minutes. Push the brake pedal down and hold it down with light but steady force. Turn the engine off and continue to hold the brake pedal down for about a minute or so. The pedal should not rise during this time.