I can tell you from having recently replaced the o rings in mine that there are many different locations that could be causing this. Could be the return lines, return caps and o rings like you are talking about which isn't too bad, could be a cracked injector line as well, could be a fuel leak from the fuel filter too. There are many places that it could be, but none of them are too bad to fix if you look up some info and most of the parts are pretty cheap too. Just typical mechanical frustrations you'll encounter. See if you have any wet spots on top of the engine or see if you can see any fuel drip from the white fuel filter while it's running and go from there to troubleshoot where you may be getting air from. Also, from dealing with this myself don't let the truck have less then a half a tank or even try to park it nose down if you can if you are driving it while these issues are going on. Anything you can do to keep fuel pressure by gravity when the truck sits keeping fuel into you injection pump will make starting the truck and dealing with the thing a little less irritating.
Mine was so full of air, I had to take the hard line into the fuel filter off from the lift pump and fill it with a squirt bottle of diesel to get things going again to get it to run.
