When the lift pump shuts off. Is there any residual pressure in the filter housing? Or does the pressure just drop back to 0 simply draining back to the tank via the return line? Does anybody have a fuel system schematic?

JMJ went off into another world i guess... he is explaining what happens when a lift pump is not working while the truck is running..

After shutting off your truck you should have remaining pressure all the way up to the injection pump (from the lift pump). That is.. as long as you don't have any fuel leaks or any way air can get into the system. Some trucks may lose their prime after sitting a few days though. But starting the truck, you most likely won't tell a difference... unless you have a leak that is.

:iamwithst

So it will hold pressure? Is there a regulator that holds the pressure in the return line? Problem I am seeing is the lift pump pressure is at 9psi and will not change even if you throttle accel. Now once it has run for about 5-10 mins then the pressure will drop in relation to the throttle. At WOT it drops to 3psi but only after it has run up to operating temps. Could the IP leak pressure back into the filter housing causing this?

my 94 held pressure today when I was testing the truck. I believe there is a check valve in the LP. I know there isn't any in the FFM cause I have extensively modded mine.

my Autozone lift pump has a ball check valve to keep the normal vacumn in the tank from sucking fuel backi into the tank. It fails some times opening the filter vent will drain the filter if the lift pump check valve has failed. I think the fuel pressure is bled through the injection pump via the pump return line to help cool the pump after a hot shutdown. Mine bleeds off the pressure pretty quick. also I will get about 1" vacumn on the gage (installed at the input to the IP ) when the check valve (LP.) fails. Auto Zone has replaced 2 LP'S over this issue no questions asked.