Vacuum Brake Booster: Testing and Inspection

Note: The following tests on the vacuum system should only be carried out if it is known that the hydraulic brake system is in a satisfactory condition.

1. Start the engine, allow the vacuum to build up and apply the brake pedal several times.
2. Check that the wheel is free to rotate immediately after the pedal is released. If the brakes bind, then the servo is faulty.
3. With the engine running, apply the brake pedal several times and check the operation of the pedal. If the response is sluggish, check the condition of the vacuum hoses and the servo unit air filter.
4. Allow the vacuum to build up and then switch off the engine and operate the brake pedal. The pedal should be vacuum assisted for approximately three applications; any less indicates a leaking vacuum system or an inoperative non-return valve.
5. With the engine switched off, operate the brake pedal several times to evacuate any vacuum left in the system.
6. While maintaining light pressure on the footbrake pedal, start the engine. If the servo unit is operating correctly, the pedal will fall under the existing foot pressure. If the pedal remains stationary, then there is a leak in the vacuum system.