Detroit 60 losing prime

I have a question.... i have a 2000 freightliner classic with deteoit 60 in it. Heres my problem, while driving or idling the truck will lose its prime. The fuel filtef on the outside of the frame rail is the one that loses fuel. The one on the inside stays full. I didnt find a fuel line leaking so we replaced it and the other fuel lines. What else could make it lose prime? Its still doing it. Could it be the check valve on the back of the head or no?

 

Well im not sure too tell you the truth. It dont have a fuel pressure gage and i cant test it.

 

We have checked all of those. Its so weird because the filter on the inside of the frame rail is always full of fuel.

 

Ok ill give it a go. Thanks, ill let you know!