Fual is going from the front tank to the rear tank when i switch to it. on 1992 Ford F-250

The fual in the rear tank can be used up, but when I switch to the front tank it goes down in less then 5 miles and will put the unused gas in the rear tank. I must us up the gas in the rear before I switch to the front tank.

Asked by for the 1992 Ford F-250