Losing Prime 6V92

[kdnelson]

I have an older coach (1976) with the 6v92 detroit that has lost its prime.
The motor was running well, though may have run dry.
I pulled both fuel filters and they were all but dry. (less than 1/4 full)
I have re-primed by refilling the filters and giving it a shot of ether to get started, which it did.
As long as I kept the RPM high it ran well. When I moved the coach or let it idle the motor died.
When I rechecked the filters they were again less that 1/4 full. First it was just the second filter then also the primary filter.
I cleaned out the strainer and blew out the supply line and filled up the primary filter and manually filled the secondary.
Again, the same scenario played out.
I am able to fill the primary filter by pressurizing the fuel tank and I got a face full of diesel.
I opened the cap on the back of the secondary filter and with the fuel tank pressurized and the secondary filter full of diesel never got fuel to the opening. I was able to get lots of air coming out but no fuel.
With the tank pressurized it was still rough at an idle.

What next?
I understand that this fuel system is a closed system and there should be no air to function.
So the only options that I have left is a bad check valve, fuel injector, fuel pump or a broken line. It would not surprise me too much if theses were the factory lines. I did not see any areas of leaking in the motor compartment
Am I missing something?
Any suggestions on what to do first, or am I in over my head?