Checking the valve (Audi A8 D2)

Unscrew the cap from the valve.

Apply some saliva to the faucet. If a bubble forms, screw the valve back on with the back of the cap. Use only the metal cap to tighten the valve.

Recheck the valve. If bubbles continue to form and the valve cannot be screwed in, replace the valve.

Be sure to screw on the cap.