Almost the same just marginally different with the car running. What valve would you be referring to?
There is your problem Dave, something wrong with your booster or the vacuum supplied to it. The valve is the plastic 90 degree fitting on the booster that the vacuum line connects to. It is a one-way valve so the booster should hold vacuum for some time even if the engine is not providing any. For example, after you turn off the car you should usually get one application of the brake pedal with power assist, after that you've lost that vacuum and no more assist.
