I agree with Ted. Full engine vacuum will not cause the booster to pull down on the pedal. The booster sees full engine vacuum when the engine is running and the check valve's only purpose is to keep vacuum inside the booster when the engine is off. It's called a check valve...not a regulator. We have to remember that he did not have this problem with his old booster....this booster is brand new and diagnostic procedures that are meant to check out a worn out booster won't work on a booster that isn't built properly. More than likely something went wrong in the manufacturing process. P.S. I'm ASE certified in brakes, my folks and I own a shop, and I teach a high school Automotive Service Technology class. Now, I'm not the loudest rooster on the fence, but I can't wait to hear the outcome on this one....If I'm wrong on this I'll just chalk it up to a great learning experience and a lesson learned I reserve the Right to be wrong at any given time