['65 chevy lover]

Howdy everyone, having problems with the fuel pump. About 6 months ago, I decided to change the fuel pump. Had it off there and then, decided to not change it( it still worked I just thoght I would start out with all new parts). Tried to start the truck and wouldn't start. It would run a little bit but never would stay running. Checked the pump and wasn't pumping. Now I go and get a new pump and, still not working. When I pulled it off this time, the push rod inside was really hard to move. Anyone have ideas what's going on? Thanks for any help.