Originally Posted by Grinnin
You are correct that it could be the diaphragm. But it could be a flapper valve.
If either valve is not working -- either not sealing or not opening -- the pump would show the symptom you describe: no vacuum on the inlet side. The pump could be simply "breathing" through one side or it could be stuck and acting as an air shock absorber.
Please excuse a post full of speculation.
That is a very good point, and probably the more likely cause.But either way the diaphram gasket needs to be replaced since they are both integrated on it.
Also, I was told a while back that Ims doesn't sell a rebuild kit, just the whole pump assembly.That seems rediculous.