Mine was getting air in the system before my more recent issue and I ended up replacing all of the return caps and o rings. However once I figured out that was what I needed to attempt, I had air seep the whole way down to the lift pump and was not getting any fuel to come up to the filter and thus into the IP.
I coasted mine nose-down on a hill and got a squirt bottle full of diesel and took the line off to the fuel filter and filled the metal line that comes out of the lift pump with diesel as it was not full to the fuel filter, and then took the fuel filter off and filled it with diesel to the brim and screwed it back on, and the first time I got in the cab and cranked it for 10 seconds I got a huge cloud of smoke and it fired fine. Once I had it running I cracked each injector open just a bit until I saw fuel to try and get any air pockets out of my system. Of course now my hard line into my IP **** the bed so I'm sure I have air again now, but you get the idea.
It also would not start with ether during this debacle, didn't really do anything with it quite honestly just a small sputter but when you have literally no fuel it's not helping anything.