The pump worked when I had a gravity tank going to it, now I have switched tanks and it won't suck. the diaphram seems good. Can i just get a new diaphram or just better to get a new pump?

FYI, the stock tank is rotten, my father had a big ol car tank in there and no seat, and I have switched to a plastic 10 gal tank out of a 77 bronco so I can have a seat.