95 Camaro – Difficult fuel pressure problem

Hi everyone. Sorry this is long.

I have an issue that is driving me crazy. Car is a 1995 Camaro, 3.4L V6. A few months ago the fuel pump went completely dead. So, I pulled the tank (real blast in these cars) and replaced the pump with a new Delphi unit. Cleaned everything up, new filter, etc. After reassembling everything, I test the fuel pressure and find on initial key on I’m getting about 12 psi (instead of 40+) and the pressure is dropping off to 0. I can get the pressure up to 30-40 psi if I cycle the key twice.

So, I test the regulator (ended up replacing it for the hell of it), check voltage at the pump connector (+ the relay) and try to figure it out. Ended up exchanging the Delphi pump for a new one, dropped the tank again, put it back together and no change.

So, next idea I have is to connect the pressure gauge at the fuel filter, then block off the pressure line with a clamp before the rail and see if I still lose pressure. It should hold(?), unless the check valve on the pump is bad and draining the fuel back into the tank. It doesn’t hold pressure, so I figure I may have found a bad group of Delphi pumps.
I just pulled the tank again, this time installing a new sender assembly (the old one was showing it’s age) and put in a new TRE brand pump. Reinstalled the tank for the third time, and it’s still a problem. Pressure is still low, and still falls back to zero. Only change is the fall back is slower than the Delphi.

What is going on here? Do no check valves work on aftermarket pumps? It’s the pressure fallback problem, but also what about the low key on pressure? I thought I may have a leaky injector or something causing that, but when you can’t even get the system to hold clamped off it’s hard to see if something at the rail is causing problems as well.

I would really appreciate any advice here. I should add that the car will start and run at the correct pressure, but it’s a little longer crank due to the low initial psi.