Ok, well I did a test. My truck loses prime overnight with the nose pointing uphill but it sat for 3 nights with the nose downhill and it held prime perfectly. One person mentioned on here that he was told by Cummins to replace the "fuel pump" and that fixed it. Does he mean injector pump? Any ideas anyone? Does that eliminate the return fuel line from possibly being the problem because I had replaced that about 2 years ago?

 

He meant the lift pump.
If the truck is parked uphill or down or if the fuel level is low when the problem occurs can indicate a leak somewhere as well as a bad lift pump.

 

I pulled the overflow valve out and took the spring/ball assembly apart and it looked visually good. Should the vavle seal air tight? If I blow on the male threaded end with my mouth lightly it leaks pretty easily.

 

GearHd6, you just described the exact conditions that mine lost prime a little over a year ago. I replaced the fuel lines (from Foster Truck) and that totally fixed the problem mine had. Like Bill said, you could have more going on there but I'd lay odds on the rubber fuel lines being the culprit. As I pointed out in another thread on this topic I also found it much easier to get my hands on the return line and clamps by removing the fuel filer canister as this really opens up the area where it resides. Plus you can replace the filter and clean any sediment from the bowl in the process.

Steve