Sequence of execution
1. Prepare the vehicle for work (see "Preparing the car for maintenance and repair").
2. With the engine not running, press the brake pedal several times until the hissing sound in the brake booster stops.
3. Press the brake pedal and hold it down.
4. Without releasing the pedal, start the engine.
5. If immediately after starting the engine the pedal moves down slightly, the brake booster is working properly.
Otherwise, we check the integrity of the vacuum supply hose to the vacuum booster, the tightness of its connection to the inlet receiver and the booster check valve branch pipe. If the hose is in good condition and is tightly connected, the vacuum booster is faulty.
[Source link - website: LadaMan.ru]