True it's rare and in my opinion power valves get the blamed for more than their fair share of problems. Not just a theoretical situation though. There is a register for the gasket on the power valve to keep it centered. I've seen the gasket hanging down where it shouldn't be at times. I've seen torn gaskets and power valves that look like somebody just forgot to tighten. Sometimes they leak. Easy to check and sounds like the Op checked it already.