My lly will not build prime pressure. The filter housing has been rebuilt and i pressure checked it with fuel outside of the truck. It held its pressure with no leaks. That was with the inlet hose in a 5 gal tank and the outlet plugged.
Inside the truck its a different story. the plunger just keeps on pumping. You can also almost hear the fuel getting returned to the tank or out a leak.(you can hear the pressure being lost) All rubber lines and clamps have been replaced between cp3 and filter housing. I tried priming it with the inlet hose for the filter housing in the 5gal tank which i put in the engine bay. So that eliminates my lift pump, tank pick up, ect...

My question is: Other than the filter housing causing no prime pressure. Could the cp3 itself be loosing the pressure, whether it be a valve or a leak?

I also noticed a T fitting on the supply just before the cp3 that connects to a return line... How does that work???

My truck is in a start then die situation please help!