As suggested you might look for one of these on the filter housing. A check valve. See picture.
I am not a fan of them as they are another moving part that can quit or clog up. They do stabilize the pulsing of weak or some fuel pumps, with the filter canister acting like an accumulator. That said, have good ones or take it out.
I read you had good delivery, good pressure. And NO bubbles to the input to the IP... Right ???
Be sure all hoses are rated diesel fuel, of high quality, tight clamps... All of them. Look in the book and find them ALL. Underneath between the tank and pump, at the pump, at the filter, at the IP input and to the purge /test valve. AND check that the purge / test valve closes OFF tight. No cheap ass, imported rubber hose from the ZONE. Name brand like Gates or....
Is the fuel canister sealing good? Might have a look there. But you say, NO bubbles...No troubles ? Did we ever LOOK into the fuel filter system ??
The small vent on fuel pump is a housing vent. If you found fuel there...the diaphragm is leaking with a good chance that fuel can find its way to the engine crankcase and oil. BAD.
One would like to get this under control as said. Stay focus. Step 1-2-3-4 from supply (tank) to IP input. IMO spend the small money, buy hose and clamps and make it all new and pretty. Don't forget return lines too. That fixed...no more worries, move on.
Another free guess. Are we...tiring to help, YOU sure the IP electrical connections are good ??
Riding along here with no real history make it hard to figure what's up. Trouble shooting is a logical approach, BUT not a real science, LOL.
Not looking to freeze my butt off, CAMO