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I know this has been answered a million times but not precisely. I do know that the check valves are bad in the front FDM but where are they located on the unit? are they in the tubing or are they in the plastic pump assembly?
They are inside the plastic pump assembly and are not serviceable even if you take it apart.. which can be done despite what Ford would have you believe.
HAHA My 88 did this. I always ran the front tank and the little canister thing on the framerail screwed up and started sending all my return fuel to the back tank. I was running low on gas and money one day so I flipped the switch just to check (knowing good and well I had run it dry) and it was FULL! I'd forget about it and drive for a week and flip the switch thinking I was about to be walking and it was like Christmas every time haha