I had my son take a 94 12v to have it aligned. They drove it after setting things up,came back made a few adjustments & it would start,but only run maybe 15 seconds & stop. It would restart if you pressed on throttle pedal alittle, but wouldn't idle but 15 seconds.

I have replaced all the incoming fuel lines w/ Vulcan's rubber lines & removed the heater from the pre filter & resealed it w/ Cummins gasket. New lift pump,fuel filter & lines to the tank. When you press the prime button you hear a sound (hiss)& feel it vibrate the line coming from the return check vavle on the inj. pump. I see/feel no leaks from it. I replaced all these things before this happening today. I Have not replace anything on the return side .

After a trailer ride home & sitting less than 1 hour, it now starts & runs even under a load ( in the driveway). The primer still make a sound from near the return line from the pump & going down in the valley of the motor

What is the return check valves function? Could it be allowing fuel to exit the pump & loosing it's prime??

Sorry for the LOOONG post

 

The valve you speak of is called the overflow valve, it's just a pressure regulator. Sort of doubt it's your problem, my bet is you still have an air leak.
Did you replace the steel lines with hose or the the rubber parts?
The steel lines are prone to leaks too, better to abandon them and run high quality diesel rated hose all the way from engine to tank.

 

I replaced all the steel lines on the feed side with rubber hoses from Vulcan. I replaced the overflow valve w/ one from a buddy & made no difference. When it does it's thing & begins to stall the throttle pedal becomes stiff .
I drove it around w/ the other valve in it & it stalls, but restarts & I can get back home. It has fuel in the filter.
A few of the local pump service shops are leaning to the pump.

 

When diesels have diveability issues the first thing to do , is a fuel pressure test .
What other mods to the truck ?
To test the lines to & from tank , install a air valve into a spare fuel cap [ tank filler ] , then put about 3 lbs of air pressure in tank , then with a flash light & mirror check ever inch of lines / hoses for fuel leaks , the supply side is under vacuum during normal operation so you would not see fuel , hard to see vacuum .
Generally never replace anything without testing , and the more the replacement part costs the more the testing .