ISPKI
Welding/metallurgical engineer/Metalsmith
If you just add a check valve between the filter header and the lift pump your drainback issues will be cured, unless you are leaking air into the filter via the fuel heater or if the old-style filter then the bottom oring.
You can also get a replacement check valve for the one in the filter header that goes to the #1 injector return line but even a brand new one wasn't anything to write home about for me. A diesel rated check valve in the supply line is more effective in my experience.
If the fuel tank doesnt have a cap on it, would that cause the fuel supply line to the lift pump to drain back into the tank? I didnt think this was the case but just curious since my front tank doesnt have a cap and I havent picked one up yet.