Losing prime overnight/fuel pressure problem

[Skman]

Did clear hose test with the fuel/water separator inline and also with it bypassed.
No air in the hose while priming or truck running.
But when you shut engine off...can hear fuel running into fuel tank. Also, the clear hose will run back and empty out.
Fuel rail stays at same pressure (within a couple hundred PSI) when fuel is running back to tank.
After turning key...it takes a few seconds for fuel to start running through it again toward the priming pump.
Almost seems like air getting into system somewhere...but there's no external leaks anywhere.