When the engine is not running, the vacuum decreases, and therefore the boosting effect disappears. Thus, for example when towing, it is necessary to apply significantly more force when pressing the brake pedal to achieve the usual effect.
You can check the correct operation of the amplifier as follows:
- press the brake pedal several times and then hold it down. If you then start the engine, the pedal should drop a little more. In this case everything is fine;
- leave the engine running. Then press and hold the brake pedal again. Stop the engine - the pedal should not rise;
- start and stop the engine again. Press the brake pedal several times. The pedal stroke should be the greatest when first pressed and should decrease with each subsequent press.
If the booster does not operate normally, it is likely that the vacuum hose is leaking or the check valve in the vacuum hose is faulty.
You can check this by disconnecting the hose from the amplifier. When working properly, a suction action should be felt at the end of the hose. When installing a new valve, pay attention to its correct position. If the amplifier is faulty, it is replaced as an assembly.
Self-repair of the amplifier is not possible, since it requires a special tool. But it is unlikely that a service station will undertake to repair the amplifier. The failure of the amplifier does not mean the complete failure of the brakes. However, the required pedal force to maintain the same braking distance will increase significantly, especially for a minibus.