Isx 870 fuel pressure

[Kiko]

Hello all again. Am having trouble with another truck being hard to start. Cranks for along time and then fires up and runs rough. No problem found when engine hot and running on highway. Lift pump is working and putting out about 15 to 20 psi. Put on clear hoses and no air found. Also with clear hose on compu fitting the fuel doesn’t appear to be leaking off to return. When it cranks after lift pump runs for 30 seconds it is slow to build pressure. After running and shut truck off. 10 seconds later it’s still hard to start. I read on here about main pumps breaking the shaft and still working but just not to their best. This could be possible but have been trying to eliminate everything else before ripping it off and replacing. A question I have is how much fuel should be returning to tank with just lift pump running. With a clear hose on compu fitting and lift pump running it flows out but not even close to what is flowing out of return hose. Thanks in advance.