I have a whisperflo pump that loses prime after the pump is turned off. When the pump is running there are a few air bubbles visible in the lid. There are no visible leaks during operation of the pump. As soon as the pump turns off you can see air bubbles very slowly pop up against the lid. The lid gasket has been replaced and greased two times by me and my pool guy. The lid clamp is extremely tight in that I have to sometimes use my deadblow hammer to loosen it. The lid and clamp are in good shape without any cracks. The pump was recently rebuilt and I took it apart again. All gaskets look good, nothing out of the ordinary. When the pump is off I hear the faintest hiss of air coming from the pump lid area. I can not seem to find the leak. Any suggestions.