Unless the valve is a little eaten up. or the springs are a little worn, then exaust gas could be leaking past it, and forcing it open. I suspected a problem with mine, So I diconnected it from the boost, and hooked it up to the vacumm pump, I figured it could not hurt, and it might help the valve stay shut, if it was cracking open under high boost. If you are making 15lbs of boost then there has to be at least 15 Lbs of exaust pressure behind that valve. It seemed to build boost faster with it set this way. Could be in my head though. The diaphram in the can is holding the vacuum just fine, no leaks I can find.
Might try it. Cant hurt.
Ken