Your problem could be the relief valve sticking closed, not open. If you cover the inlet, the relief should open and let in some air. Your vacuum should not go over 14" hg. If the vac goes way over 14, and you don't hear air sucking into the relief when you cover the inlet, then it's stuck.
If the relief opens and the vac stays at or below 14, then your problem is something else.
If my suggestion doesn't make sense, then it's probably time to visit your local distributor and let them have a look at it.