I realize there is 1000 threads on this, but thought i'd add one more. I've had a hiss at my feet since i bought this thing. Yesterday replaced check valve, and still the hiss. I can't imagine my booster is bad because brake pedal reacts normally...meaning not soft, not hard to press, just normal. I have a vacuum leak, that's a given, but where is it? the hissing disappears when the either the brake or gas pedal is depressed. The only thing it really affects that i know of is my idle. i'm right at 1200rpm now, but if i push the brake in and stop the hiss i drop down to about 800-900rpm. much appreciated in advance for help