After driving my 91 F150 for about 5 years only on the front gas tank I finally decided to fix the rear pump. Upon disassambly, I found, to no real surprise, a rusted gas tank and a rusted sending unit. I will add that the tank was full of dead smelling gas. I don't believe that I would have ever filled the tank up because I rarely ever do. I needed to move the truck before reassembly so I switched the tank selector switch on the dash to front tank to drive out of the building and noticed that while the truck was running gas came out of the supply line at the rear connections. At reassembly with a new gas tank and a new oe equivilent Bosch pump AND the old external check valve when I now drive off the front tank, the rear tank fills up with fuel. I can only go approx. 20 miles on 6 gallons before the front tank is empty and all of the fuel is in the rear tank. I have replaced the rear pump with another Bosch pump and a NEW external check valve from Ford with the same problem. Before installing the second pump and check valve I air tested the check valve and it will hold at least 60# of pressure from the output side. I am lost as to what to try next. Any help would be appreciated. Thanks.