Fig. 6.3. Checking the operation of the vacuum booster
Run the engine for one or two minutes and then turn it off. Press the brake pedal several times with the usual force when braking. If the first time you press the brake pedal, it is pressed all the way down, and its travel gradually decreases with subsequent presses, the vacuum brake booster is working normally. If its travel remains unchanged with successive presses of the pedal, then the vacuum brake booster is faulty (Fig. 6.3).
With the engine off, press the brake pedal several times and make sure that the brake pedal travel changes. Then, without releasing the brake pedal, start the engine. If the brake pedal drops slightly, this means that the vacuum booster is working properly. If the pedal remains in place after starting the engine, then the vacuum booster is faulty.
Fig. 6.4. Checking the serviceability of the vacuum booster
With the engine running, press the brake pedal and then turn off the engine. Keep the brake pedal pressed for 30 seconds. If the pedal position does not change, the brake booster is OK. If the brake pedal rises, the brake booster is faulty (Fig. 6.4).
If the results of one of the tests performed are unsatisfactory, you should look for a fault in the vacuum hose, check valve or brake booster.