Is it possible to have a pretty good leak in your power booster & still have it perform well? When I hook up my power booster my engine runs rougher & I can (seem) to hear a hissing sound. Its hard to tell over the sound of the engine. I have done the break pedal test where you turn off the engine while holding the pedal down. It seems to pass the test okay. Thanks

There is a cup seal between the booster and master cylinder, the master cyl vents to a cross hole underneath and right up against the booster. Give a listen there, use a piece of hose up to your ear to find it. The seal is pretty easy to change if that's the problem, but I don't know where you'd find one.

Yes it is possible. I just replaced mine and it seemed to go bad over a period of time and slowy stopped working. Man what a difference when I replaced it. The brakes seemed to work ok but gradually got worse.