I watched a Stanadyne video on our pumps, and now I am totally confused on how a leak in the return line causes trouble. Near as I could make out from the Stanadyne video, the fuel comes in from the supply line, and that's all there is to it. How would air in the return lines be a problem? Is it because the return lines are hooked to the fuel filter, so the pump sucks return air through the filter? Do the return lines HAVE to be hooked to the filter?
Edited to add: the problem is that the return line leak lets the supply side empty back into the tank, right? So removing the return line to fuel filter connection should help a lot. Can we do that?
Most people have taken their straw at the restaurant, put it down in their drink, put their thumb over the end, and have pulled the straw out of the drink and watched the drink stay in the straw, hovering in place as long as they keep their thumb over the end of the straw.
That is how the fuel system in these trucks work. The fuel pump pumps the fuel from the tank up to the top of the engine, filling the filter, the injection pump and eventually the return system. When the engine is turned off, the fuel should stay or "hover" in the fuel system on the top of the engine. If the system is totally sealed at the top, the fuel will stay up there, just like if you have your thumb sealing the straw. Any little air leak will let the fuel start migrating back down the return line to the tank.
You can tell sometimes how bad the air leak is. If you stop the truck just for the night, and it stalls in the morning and takes a lot of cranking to get it going, you have a fairly large air leak somewhere. If you have a smaller leak, it will not take as much cranking. If you have a very small leak, it may have to sit a couple of days before it gives you trouble.
Don't forget about the heater on top of the fuel filter housing. If there is any wetness around that area, it is leaking and letting air in, it's a common problem area also for air intrusion. Look the fuel system over carefully after running the truck for a week or so, any wetness is a clue there is a air leak there.