If my F150 has the front tank selected it will return the fuel to the rear tank (or vice versa...it's been a while since I've drove it). It can cause an overflow situation if you don't watch it. There were a few TSB's documenting this problem...and there was a fix that Ford did. Before you procede you may want to see if your pickup had those problems and if it was fixed. I believe you can give your dealer your VIN number and they should be able to look it up.
Here are two TSB's that I've found on it. There may be more. I know I've heard of guys with '94 F150's with the same problems.
#1
NHTSA Campaign Number: 91V146000
Vehicle/Equipment Make: FORD
Vehicle/Eqipment Model: F150
Model Year: 1989
Mfg Campaign Number:
Mfg Component Desc: FUEL SYSTEM, GASOLINE:STORAGE:AUXILLARY TANK:SELECTOR DEVICES
Mfg Involved in Recall: FORD MOTOR COMPANY
Manufacture Dates: 02-01-1989 through 02-01-1991
Type of Report: V
Potential # of Units Affected: 350,000
Date Owner Notified by Mfg: 12-20-1991
Recall Initiated By: ODI
Mfg Responsible for Recall: FORD MOTOR COMPANY
Report Recieved Date: 09-04-1991
Record Creation Date: 09-30-1991
Regulation Part Number:
FMVSS Number:
Defect Summary: THE DUAL FUEL TANK SELECTOR VALVE MAY MALFUNCTION AND CAUSE AN OVERFILL CONDITION IN ONE OF THE FUEL TANKS, RESULTING IN AN OVERFLOW OF FUEL PAST THE FILLER CAP.
Consequence Summary: THE OVERFLOW OF FUEL PAST THE FILLER CAP CAUSES A FUELLEAKAGE, WHICH CAN RESULT IN A FIRE WHEN EXPOSED TO A SOURCE OF IGNITION.
Corrective Summary:
#2
NHTSA Campaign Number: 93V125000
Vehicle/Equipment Make: FORD
Vehicle/Eqipment Model: F250
Model Year: 1993
Mfg Campaign Number:
Mfg Component Desc: FUEL SYSTEM, GASOLINE:STORAGE:TANK ASSEMBLY
Mfg Involved in Recall: FORD MOTOR COMPANY
Manufacture Dates:
Type of Report: V
Potential # of Units Affected: 1,131,000
Date Owner Notified by Mfg: 10-28-1993
Recall Initiated By: ODI
Mfg Responsible for Recall: FORD MOTOR COMPANY
Report Recieved Date: 08-02-1993
Record Creation Date: 07-27-1993
Regulation Part Number:
FMVSS Number:
Defect Summary: A MALFUNCTIONING CHECK VALVE WITHIN THE FUEL PUMP ASSEMBLY CAUSES A PORTION OF THE UNUSED FUEL FROM ONE TANK TO BE RETURNED TO THE SECOND TANK. WHEN THIS OCCURS, THE CAPACITY OF THE SECOND TANK CAN BE EXCEEDED, CAUSING FUEL SPILLAGE.
Consequence Summary: SPILLED FUEL CAN RESULT IN A FIRE WHEN EXPOSED TO ASOURCE OF IGNITION.
Corrective Summary: