I have a 94 K1500 with a 5.7. I checked my fuel pressure the other day, when I cycle the key I have 21 psi but as soon as the pump shuts off the pressure drops off to zero in under 10 seconds. i did notice a very small drip from the injector when pressurized so I replaced the o-rings on the injectors and I also replaced the PR diaphragm. Tested it again & drip is fixed but pressure still drops off when the pump shuts off. I have checked all over this truck but I do not see (or smell) a leak anywhere. The pump will maintain pressure with the engine running. I replaced the fuel pump & sending unit about a month ago...installed an AC Delco pump, EP377. So my thoughts are:
1) Bad fuel pump? I assume there is a check valve in the pump?
2) Leaking off inside the tank (I didn't splurge on the sending unit...I think it is a Spectrum).
3) Do I need a heavy duty pressure reg diaphragm for the higher fuel pressure? The one I put in is just your standard rebuild diaphragm?
4) Bad fuel metering body or cover (injector pod)?

I don't want to pull the tank again if I don't have too but that's the way i am leaning. Any thoughts? Thx.