I would almost be willing to be that when you pulled the fuel pump to replace the sending unit, you developed a crack in the corrugated hose from the pump to the module outlet fitting. That will drop fuel pressure and drive you bonkers. Ask me how I know.
The check valve in the pump itself is just a ball bearing. That hose gets brittle with age and once it comes out of the fuel bath and dries out, you don't even want to look at it funny or it will crack.
Glad you got it fixed though.