2006 jd325 no start

[jggriffith005]

I have a 2006 jd325 straight skid steer that started off a few months ago by just sputtering at idle and when you'd give it a little throttle it would level out, then it began to not start due to a pressure actuated fitting on the block for the return line failing, I would start it with vice grips on the return line until I got the new fitting replaced, it worked for a month or so then wouldn't start at all. It would just free spin, We flushed the diesel, changed the lift pump and fuel shutoff solenoid then it began to actually crank and sound like it was gonna start. I called my local parts shop and they have never heard of a fitting on the return line that has anything to do with pressure, so we found a 10 psi aftermarket one and it wouldn't start, we put a 20psi on it and now it starts somewhat hard after priming the system every time but it only runs for about 10-15 seconds and dies. It starts easier with vice grips, and without the pressure fitting it just cranks. The fuel lines are new, no air leaks that I can find. I am stumped and would appreciate any help.