When was the last time the rear tank was used? If it hasn't been to long, i guess you could switch the top two fuel lines with the two bottom fuel lines on the fuel selector valve to see if the front tank works?? Top two lines or for the front tank.But if the fuel selector valve is bad it may not allow fuel to your main external fuel pump, and you could burn it out. On my 87 f150 4.9 efi my fuel selector valve was bad and would not allow fuel from my front tank into the engine. All the fuel from my rear tank would get pumped to the front tank in about an hour, then when i switched to the front tank it stalled. Have you noticed fuel switching tanks, or overflowing or build up of pressure when you open the fuel caps? Recall for for 89's which had the same part, i guess because of the age of the truck the part is old.The valve cost about 65.00 at fords, i could not find a aftermarket mechanical ( no wire to it) valve. In your cause it may not be the problem. How many mpg do you get on your front tank, 19 gallon front tank at about 13 mpg is about 250?? If you get about that, then the selector valve is maybe ok?? When did your problem start? Sounds like your truck runs ok besides that? Maybe you could put some mystery oil in the front tank to keep the fuel from going bad??