Yeah thats what I thought. The only thing that was throwing me off was the fact that its noticeably different when the motor is off but that makes sense that the check valve would be bad since its bad right when the motor is turned off.
I suppose if you really wanted to....you could just replace the check valve for a quick and easy check. Or you can put vac. to the 'booster' side of the check valve to see if it holds. If it holds vac. then that's not the problem. At that point I would say booster.
