I agree. If the brake booster has a leak in the diaphram, then it will drop idle & stumble when you hold your foot on the brake while in gear. It will run normal otherwise. You'll often also hear an "airy" sound when you step on the brake.. but that's not always as obvious. It depends on the size of the leak. Also, its usually when you've been holding the brake for a few moments before it starts to idle down, but I suspect that if the leak was bad enough it could start doing it right away.
