Fuel Problem Cummins

[Therealarw]

Hello everyone,
I've been having this problem that I haven't been able to figure out for over a week now. I have a 2014 Pete with a Cummins ISX15 CM2350. When I go to cycle the key switch on to turn on the electric lift pump, I notice that the fuel filter on the outside of the frame will syphon down to about the bottom of the fuel filter, rather than staying up and full. The truck will run still but then it'll eventually shut off at idle. What I noticed is that when the lift pump is going for the 60 seconds and I open up one of the fuel tank caps I can see the fuel going out the return pipe on the bottom. I've replaced the high pressure relief valve on the end of the fuel rail. It's still doing the same thing. I'm completely stuck on this. Is there anyone who has dealt with this same thing that could steer me in the right direction? Thank you!