1. Check the tightness of the connection of the vacuum hose with the fitting on the receiver..
2.... and with a check valve.
3. To check the check valve, loosen the clamp and disconnect the vacuum hose from the valve. Remove the valve from the booster housing.
4. Put a rubber bulb on the fitting installed in the vacuum booster and squeeze it - the air should exit through the valve.
5. Release the pear; if it remains in a compressed state, then the valve is working. Otherwise, replace the valve. In the absence of a pear, you can purge the valve with your mouth.