Diesel Bombers - Losing fuel pressure

So I will start with some info

2001 CTD, Carter 12v lift pump mounted under bed, 3/8 fuel line from tank to lift pump and on to fuel filter, fuel pressure gauge reading taken from IP inlet.

I noticed that over the past couple weeks the fuel pressure was dropping a bit. Also the lift pump was quieter then normal. It started dropping below 5psi at hard peddle yesterday.

First thought was that my lift pump was failing. So I first changed the fuel filter thinking maybe it was clogged a bit. That changed nothing.

So I installed a new carter lift pump in the same location. I had better pressure after a couple minutes of running the pump. Then it dropped off again, even lower then before. Now it will not maintain pressure at all when the engine is running. It will get up to 10 while engine is off (still lower then normal used to hold 15 psi at idle, 10 at WOT)

No leaks anywhere, have lots of fuel pumping rite to filter. Engine starts and runs fine, but I really don't like the fact that my fuel pressure is low.

Now here is one question that comes to mind, could this have anything to do with the pressure valve on the return line from the IP? could it be stuck open letting the fuel run free not building any pressure? Or maybe an air leak somewhere between the tank and lift pump?

any advise would be appreciated

Kinda stumped here, trying not to just start swapping stuff until I find the issue.

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Ha just realized this was my first post here. I have been on the site for a while, just reading I guess.