Hy Iron Clad, if you have no vacuum reserve after the engine is shut off, then you probably have a leaking check valve or other leak. Does your engine run alright, consistant at idle? I'm not sure if you understand how the vacuum assist works, so I'll give you a brief explanation, under normal driving conditions there will be vacuum on both sides of the booster diaphragm, when you apply your brakes an atmosphere valve is opened on the apply side of the diaphragm, this allows air under atmospheric pressure to enter, this air is higher pressure than the vacuum on the other side of the diapragm, so the diaphragm and the piston attached to it will move, pushing brake fluid out to the wheel cylinders or brake calipers. The more you press on the pedal the more air is admitted through the atmosphere valve, and the more brake apply "power" you get. So monitoring vacuum will only give you limited information. A quick test to see if your vacuum assist booster is functioning correctly is, with the engine off pump the brake pedal a few times to make sure all the reserve is gone, now press and hold the brake pedal down, and start the engine, when the engine starts the pedal should go a little closer to the floor if the booster is functioning correctly, hope that helps.
