Fuel bleeding down

I have a 923a2 and am having some issues with fuel pressure bleeding off. The truck used to pop right off every time no matter how long it sat. However now it seems like if I let it sit for a few minutes, it has to turn over for about 30 seconds in order to start up. If I shut it down and restart it immediately it will crank right up. I had an off brand lift pump on it which I thought was the issue but I just changed it to a Cummins unit and the same thing happens. Is there a check valve somewhere that could be bad? Thanks

Pinholes in the rubber lines. They won't leak fuel out, but they'll allow loss of prime.

Check fittings at the pump.

Yes there is a spring loaded pressure relief valve (ball) in the outlet banjo fitting bolt on the injection pump. Common air entrance points are soft suction lines, fuel tank dip tube, solder at top of fuel tank dip tube fitting, square washer mis-seated at top of frame mounted filter if it has one, etc......some mentioned here already.

Because of the overflow valve and the 2 check valves in the lift pump, fuel should not bleed back from the fuel pump to the lift pump. Usually if there is a leak before the engine, the engine will fire, idle for an bit and then shut off, it will take lots of cranking to re light it. Since you have replaced the lift pump twice, I would either put gauges and a clear hose from the filter to the pump on it and see what happens, or, pull the trigger and buy a new overflow valve. Or, you have crap in the fuel system and it is getting into the check valves in the lift pump(s).

Here's a quick diagram of how to install a clear line and tell whether or not it's before or after the pump where you're losing your Prime. When you install the clearline make sure it either has a loop or a big hump in it. If the fuel drains back from the fuel pump side it's the check valve, if the fuel drains back from the filter side it's either a lift pump or a poor connection somewhere between the lift pump and the filter