About 15 days. I hear ya about the mechanical pump. I didn't think there was a true check valve inside the mechanical fuel pump. I did replace it with a new Carter pump in 2019. Not that replacing anything with a new part means anything of course!!!How long was Colonel parked?
Methinks you have bad check valves in your mechanical pump.
Seems you're making nice progress with the issue. Agree that it shouldn't be happening because of the mechanical fuel pump. Believe you're on the right track that somehow the fuel drains or gets sucked back into the tank.Not that replacing anything with a new part means anything of course!!!