I replaced intake manifold gasket on this 4.3 Vortec as coolant was leaking into crankcase. In the meantime I changed out the fuel filter too. Got everything all together, connected battery, and ready to fire up engine, I turned on key to prime fuel pressure and WOW, fuel started dumping out of fuel tank as it sounded like it was being pressurized, can hear air hissing out of it, then the fuel, so I disconnected the outlet side of the filter and turned on key/pump ran and fuel came out of filter and not out of the top of the fuel tank this time, looks good so, I repeated and reconnected fuel filter line. Tried again, fuel tank gets pressurized and spews fuel. I suspected a blockage? to engine so I depressed fuel pressure port pin and turned on key again, got fuel out of port but still coming out of tank! I pulled the lines going into the injector and got fuel coming out of them but the damn tank keeps pressurizing and dumping a lot a fuel...so what is causing the fuel tank to get pressurized that fuel is pouring out from the top vent and lines to engine appear to have no blockage?

My luck would have it when the fuel filter was replaced, the pressure line was twisted just a little bit when tightening and a rusted out part cracked against frame rail, dropped tank to repair it and broke vent port out of pump top, wait it gets better...I stepped on brake pedal and heard air whoosh out rear end as the pedal went to the floor, yep another rusted out line to replace. I guess I should count my blessings these all could have happened while traveling, I'll soon enough get to see if the intake job was worth it!

Rust is a pain!
I am glad you figured out your issue, thanks for sharing, sorry you have had so many issues at once though!