Fuel problems
 

Hello, I have an 1989 f150 with 5.0 EFI. I have had fuel tank pressure problems when I got the truck I was told that the sending units in tank were new and that he only used the front tank. so I went to the station and filled up both tanks. I drove 5 miles from station to house and see fuel leaking from front tank. I tried to take the gas caps off to release pressure. The switch on dash was on front tank and after removing the caps the rear tank was overflowing, so I returned the cap on rear tank and tried to use that tank to take the level of fuel down and then I was getting gas leaking from the top of the tank. so I pulled the bed off and all lines were hooked up. so after doing some research I have found that the truck has a pressure tank selector and that could be bad but I haven't changed that yet and also found that one of the sending units could have a check valve bad. But the previous owner said they were new but still not sure. so before I go buying parts that I don't need I am asking for help?