How to Test Brake Boosters

Power brakes represent one of the greatest advances in automotive braking technology. Not only do power brakes make it easier for the driver to stop the vehicle, but without the assistance of the power booster it would be practically impossible for disc brakes to be effectively used on automobiles. Any deficiencies in the performance of a power booster can seriously impair braking effectiveness, and it is important to be sure the booster is operating properly at all times. Testing the booster is a simple job that every driver can do.

Procedures:
1. Turn the vehicle engine off and pump the brake pedal a few times to deplete any remaining vacuum in the power booster. Push and hold the brake pedal down with light but steady pressure. Start the engine. The brake pedal should drop slightly, indicating that the booster is working properly.
2. Allow the engine to run at idle for a minute or two. Turn the engine off and slowly pump the brake pedal five or six times. The brake pedal should stop at a higher position with each pump, indicating that the booster is not leaking and the check valve is working properly.
3. Start the engine and allow it to run at idle for a minute or two. Push and hold the brake pedal down with light but steady pressure, and turn the engine off. Continue holding the pedal down for 30 seconds or so. If the pedal does not rise it indicates that the booster is not leaking and the check valve is working properly.
4. Test the vacuum booster check valve. Locate the vacuum booster. It is a dome shaped assembly mounted at the rear of the engine compartment on the driver's side. Now locate the flexible vacuum hose that runs from the engine intake manifold to the vacuum booster. Remove any retaining clips that hold the hose to the intake manifold connection, and slip the hose off the connection. Try to blow into the hose. If the check valve is working properly the air will not flow into the booster. Now try to suck air out of the hose. If the check valve is working properly air will flow out of the booster.
5. Connect an engine vacuum test gauge to the connection on the engine intake manifold. Start the engine and let it run for a minute or two. Check the test gauge vacuum reading. The vacuum should be at least 18 inches.