I've got a friend with a mid 90's Chevy van with an intermittent brake problem. The power brakes lose their power assist and leaves a very hard high pedal. Then for no reason the assist returns and it stops fine. The van sat for 3 days and when I pulled the vacuum hose off the booster hissed just like normal. Holding vacuum just fine. With the engine running when you press the brakes the engine speeds up briefly and then returns to normal. If you unplug the hose the engine speeds up and when you plug the hose with your finger the engine returns to its normal idle. I've had master cylinders go bad and leave a very low pedal but this pedal stays very high. Is there a valve in the booster that could stick? I don't want to tell him to replace something without being sure. Do boosters go bad and still hold vacuum? Thanks.