The diaphragm in the booster can leak while still being able to help apply the brakes. You can verify it is leaking by observing the brake pedal right after you stop the engine. It should hold vacuum for at least two power assisted applications long after the engine is stopped. If the pedal gets hard to push as soon as the engine stops, either the booster or the vacuum check valve is leaking. To test the check valve, pinch the vacuum hose going to the booster while the engine is still running. Now if you have two or more power assisted stops, replace the check valve.
It's more likely to be the booster because the check valve dosn't do very much while the engine is running and it won't cause rough running. Only the booster has to be replaced. If you don't unscrew any of the steel lines from the master cylinder, you won't have to bleed the air out either.
Saturday, May 21st, 2011 AT 10:40 PM