1. When checking the serviceability of the valve, remember that its function is to pass air in only one direction, namely from the vacuum booster unit. If the valve allows air to flow in both directions or neither, it is defective and must be replaced.
2. To check the valve, disconnect the hose from the vacuum booster unit and blow into it - air should pass freely through the valve. Now try to sharply draw air into yourself through the hose - the valve should not allow the slightest leak.
3. If the valve is defective, replace it (see chapter 7).