Runs fine loaded under a hard pull, and once it is running all things seem to be fine.
If you shut it off and leave it sit for say 1 minute it has to crank for say 30 to 45 seconds to get fuel pressure to start.
OK First thing i would do is get a wrench and tighten every fitting haveing to do with fuel making sure your not suck air . CK all the fitting at engine and also around the fuel tanks. To me it sounds like you have an injector going bad Causing the fuel pressure to drop when unit isn't running. The best way to find out is run your truck by the nearest freightliner and have them run a diagnostic ck, have them run a cylinder cutout test to determine which injector could be acting up. They should only charge you 1 hours labor. And if you want to ck to see if your truck is sucking air you can install a clear line in between the fuel filter fitting and the line itself if there are alot of bubbles present you have a line leaking causing your engine to suck air and drop fuel pressure when unit sets.
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Injector balance check shows no problem and all lines from check valve foward have been checked and no problems have been found. To me the lines behind the check valve do not seem to be a factor in this problem.
Hmmm well sounds like your loosing your prime somewhere, Did you ck your cross-over fittings above the transmission. And has a clear line(site glass) be used on the unit yet.
Also away to ck and completely elliminate the sucking air possiablity is to make a fuel line and run it from the back of your fuel pump where the fuel supply goes and run it into a bucket of fuel and that will tell you if it is sucking air. ALso you even tho the pump is new doesnt always mean its good. I've myself have put many new parts on and they are bad. I would have all the pressures reckd at the pump to make sure that isnt the problem aswell. Your completely right about the ck valve it should keep fuel from draining out of the head, ALso if there has been any trash in your fuel system it might have went through your head and gotten suck in the ck vavle again and also if the ck valve has an oring it might have gotten torn or rubbed during installion causing this problem aswell.
Hope something i gave you helped at all, And just keep in touch and tell me how it works out, and if there's anything else i can assist you with plz fill free to ask.
No sir the return line doesnt hold any pressure it just returns fuel back to the tanks
One more thing check your cam shaft sensor, The sensor has alot to do with the delivery of your fuel. ALso there is a return ck vavle in the back of your fuel pump have you ckd or replaced that one.
I hope this has helped you out