1988 Ford F-150 gas tank

The truck has duel tanks, if they are both full pressure will force the gas out of the ft tank through the filler neck---if the rear tank has gas in it and the ft tank is low, the rear tank will bleed over into the ft tank. Also the switch that controls the switching of the tanks from ft to rear will work properly on the rear tank--- but when you switch to the ft tank the gauge will register full when the tank is empty. Any and all help would be greatly appreciated. Thanks