The valve I was referring to is inside the booster, not the vacuum check valve. That vacuum check valve can cause what you're describing too, but there will be other symptoms with it. One is you'll have normal power assist while the engine is running or decelerating during coasting, but even a slight acceleration will lower vacuum enough that you won't have much assist. That valve is supposed to store enough vacuum in the booster for two or three power-assisted stops during periods of low vacuum, like when accelerating. To test that valve, you have to start with normal power assist, stop the engine, wait perhaps ten to twenty seconds, then apply the brakes. You should still have power assist for at least two pedal applications. If the check valve is leaking, you will have no power assist as soon as the engine stops.
Thursday, November 12th, 2015 AT 7:10 PM