1989 ford f-150 5.8 Fuel Pumping Question?

I just picked up this truck .sat in barn for 2 yrs. seems to run good. except when I filled up the two tanks at the gas station today all the gas pumped out the front one .. I have no clue I switched from front tank to back tank still pumped all gas out front gas spout didn't matter witch tank I had it switched to. I went under and tapped on the thing all the gas hoses go to on frame just kept pumping any idea? Thanks in advance.