I see that you have not received an answer yet. I had a dual tank Diesel which had a switch under the dash to change tank feeds. Both tanks reported fuel amounts to the same fuel gauge as each tank was switched on.
If this is the system you have (using gasoline) then simply turning the dash control should isolate the rear tank. If yours is different, you need to look underneath the truck at the fuel line routing.
For gasoline engines there was an idea to meet EPA Emissions standards by closing the fuel loop. This meant that unused fuel that was brought to the engine was returned back to the fuel tank. Some things were different for Trucks by weight Class and because of the EPA rules, trucks keep their carburetors while cars went to fuel injection. The "light" Trucks changed to fuel injection before "heavy" Trucks.
Most of the fittings to fuel components can have a taper machined to match fuel components only. There can be different thread seats, even Metric for replacement parts. Your best bet to match components is disconnect the problem part, take it with you, and then get a cap plug to block the flow. This is providing that the rear tank does not need the loop anymore.
SOURCE: Fuel leaks from rear tank to front.
The leaking from one tank to another was covered by a recall from Ford. Not sure if they still foot the bill for installation of the required check valves. These are still available from the dealership. The rear one is the hardest to install. The check valve can be installed without pulling the rear tank. As for your front sender I would clean all of the connections to the sender and make sure it has a decent ground. If it still only reads at the 1/4 tank level you will have to replace the sender to rectify the problem.
SOURCE: F150 dual fuel tank problem
I have to say i'm only guessing, but it sounds like the valve that switches the flow of gas from one tank to the other must be sticking part way open, or bypassing or something, and anything your engine doesn't require, is being returned to the front tank, instead of the rear.
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