Attention: TDR Forum Junkies 
Report on my recent loss of prime problem:
After changing fuel hoses, inspecting the fuel heater for leaks, checking the overflow valve and tightening all the banjo fittings, it still lost prime in less than 3 hours. I figured whatever it was had to be a big leak. Using my Cummins engine manual flow chart, the only thing left was the lift pump. Mine had a slight leak at the primer button but a new "O" ring fixed that. It still lost prime. I went to the Cummins dealer and for $150 I got the latest style 12 Valve Lift Pump. (Cummins P/N 3936316 for those who are sure to ask) It's made by Carter in the USA and is quite a bit larger than the original that was made in England. That fixed my problem. I think the check valve in the pump must have gone bad and every time I turned off the engine the fuel flowed backwards into the tank. When I took off the fuel filter prior to changing the pump it was only half full. Another side effect of the new pump, my idle is back to normal, about 800 RPM in Drive with the AC on. It had been running about 650 and raising the idle speed was on my to do list. I guess the pump was bad enough that it wasn't pumping enough fuel at idle speed to keep the RPM's up. It didn't seem to have any power loss on the road though. Anyhow, I thought you might be interested in my fun and games and how it turned out.
After changing fuel hoses, inspecting the fuel heater for leaks, checking the overflow valve and tightening all the banjo fittings, it still lost prime in less than 3 hours. I figured whatever it was had to be a big leak. Using my Cummins engine manual flow chart, the only thing left was the lift pump. Mine had a slight leak at the primer button but a new "O" ring fixed that. It still lost prime. I went to the Cummins dealer and for $150 I got the latest style 12 Valve Lift Pump. (Cummins P/N 3936316 for those who are sure to ask) It's made by Carter in the USA and is quite a bit larger than the original that was made in England. That fixed my problem. I think the check valve in the pump must have gone bad and every time I turned off the engine the fuel flowed backwards into the tank. When I took off the fuel filter prior to changing the pump it was only half full. Another side effect of the new pump, my idle is back to normal, about 800 RPM in Drive with the AC on. It had been running about 650 and raising the idle speed was on my to do list. I guess the pump was bad enough that it wasn't pumping enough fuel at idle speed to keep the RPM's up. It didn't seem to have any power loss on the road though. Anyhow, I thought you might be interested in my fun and games and how it turned out.
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Easy fix.
