got a 66 skylark, checked the booster with my mighty vac tester, should the vacuum booster hold vac or am I reading a small leak, holds psi but no vac? I know there is a valve in there not sure how it works!
I believe there is inside the unit a diaphragm. When you hit the brakes, Atmospheric pressure is introduced on one side of the diaphragm and vacuum is present at the other side. This moves the diaphragm and provides your boost to the master cylinder. When you let off the brakes, Atmospheric pressure is removed and the diaphragm returns to rest position. So the booster should hold vacuum when brakes are not applied, yes. I believe it does have a check valve that will not let air INTO the booster from the engine side, so this may be why you see pressure being held? If you have a vacuum leak, I suggest checking the rubber seal between the booster and the master cylinder. I had this go bad and lost power assist.
Yes it should hold some vac when pumped with the mighty vac.....I diagnosed a defective brand new booster that way with my car last year.....J
