allow vacum to be stored in the booster so you have some assist for a stop or two if the engine stalls while driving, If the check valve is working correctly you can push the brake pedal down a couple of times with the engine off and you can feel the boost go away as you use up the stored vacum. The pedal gets harder and higher.


Quote Originally Posted by Rus
That is in fact a check valve. It allows for the vacuum from the intake manifold to operate the brake booster. It's purpose is to prevent vacuum fluctuation in the manifold due to you applying the brakes. The extra nipple is a known vacuum leak source and thus shouldn't be disturbed. HTH