This will be a long story, but trying to get all the information that I can here in one post instead of a full thread of back and forth.

My 93 single cab short bed 4.9L I purchased almost a year ago from my FIL. He had a shop replace the rear fuel tank. I've had to fix other issues that this shop created previously with the transmission and using ultra grey on the top cover and way too much, plugging up the oiling passages, causing a bearing to fail. On the gas tank, instead of getting a new vent vale gasket, they used ultra grey again, which isn't approved for use around gasoline. By the time I looked at it with a mirror and flashlight, the vent was about 2" above the tank and the gasket stuff looked like chewing gum stretched out. I had previously dropped the tank and replaced the vent valve with a new one and new gasket.

Then I started noticing worsening throttle response on the rear tank. Got a fuel pressure loaner kit from the store and was getting lower pressure on the rear tank than the spec allows for, and the pressure would drop off on a pull, but on the front tank the pressure would increase slightly. I was doing the measurements at the fuel filter, not the fuel rail due to issues with the schrader valves., so dropped it and sure enough there was dirt in the tank and enough that it had clogged the intake sock on the fuel pump. New Delphi fuel pump assembly, the whole thing with a new sender. Also cleaned out the tank as best I could. Flushed it with water for quite a while, used a marble/giggle siphon to remove the settled debris, then dumped it all out, used some shop towels and tilted the tank back and forth to get out as much water as I could. Used a box fan blowing on it all night to dry it the rest of the way. Installed the new pump and reinstalled the vent valve. Reinstalled in truck. Checked the voltage at the connector both with the engine off key on and with the engine running and I'm getting good battery voltage at the pump.

However...No matter how many times I cycle the key when the rear tank is selected it won't build enough pressure to allow the truck to start without me pumping the throttle. Also, the engine starts bogging down when I drop to about 3/8 of a tank. Front tank doesn't have either issue. I know I probably need to check the fuel pressure with the rear pump, but I haven't gotten a loaner for it yet.

So, thoughts on what I should do next or what could be the problem? Probably a fuel pressure test next, but that will just say what I already know, that I'm getting lower pressure on the rear tank than what I should be getting. I changed out the fuel filter when I got the truck just under a year ago (and less than 3k miles ago).

 

Pumping the throttle has ZERO effect on how much fuel is delivered. That is a red herring.

Fuel pumps for these trucks are very suspect. I have not found a brand yet that works every time and also lasts. It's a crap shoot, sadly. If the pump was starved for fuel due to the plugged sock, it probably killed it.

I recommend you just buy a fuel pressure gauge of your own.