Forgot one thing to ask. Do you have a filter between tank and pump? If so, take it off and try.
It will take more time to test vacuum and all that, than just doing my isolation/elimination procedure. In my opinion, You can do component testing once you isolate the problem. Pick which way you want to skin this cat. Either way is useful.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn