Lets see if my diagnosis is correct..............100 degrees is the killer.

[mattplumber]

Well since you know these pumps better than almost anyone, is there a way to repair the leak on the advance piston without doing a full rebuild? I only ask because the truck runs great otherwise. Good power, accelleration, fuel econ, etc. I just cant warrant a 500-1000 dollar rebuild if all thats really wrong with it is a 5 dollar o-ring or something "silly" like that. But I also understand that if it is necessary to tear down the whole pump to replace that worn component then denying a rebuild is just foolish................

 

[Agnem]

I could probably get it repaired for you, but I wouldn't consider that a good value. By the time you get it off, send it in, get it fixed, and sent back, you'd be probably a couple hundred in the hole, with nothing new to show for it.

 

[Shadetreemechanic]

Throw a clear line between the number one injector and the filter head. Then you can at least verify that it is air intrusion. If it is air then you can look for cheaper solutions than the IP before going there.
I would hate to see you replace the ip when the leak was a loose connection at a return line.