I went to pep boys and got my car diagnosed 90 chevy convertible 5.7. They said was the it the fuel pump and the sending unit, I replaced both of these but now it is losing pressure from somewhere, I think it coming from the locking cap or fuel tank might be leaking. Is there a pressure test you can perfrom on the tank to find out which one it is losing pressure?

 

not in the tank, check your fuel pressure regulator, fuel filter. make sure the fuel pump is actually pumping and try and see if your lines could be clogged. how did you realize you are losing pressure? Your injectors might not be opening all the way causing a restriction and drop in pressure.