I drive a '94 F-150, 197,000 mi., several months ago it began to spill fuel from the front tank fill . I thought I was overfilling . I quit using the front tank after running the fuel out and just used the rear tank . Well the front tank again spilled over and was noticing I was running out of fuel from the rear quicker . I think somehow the fuel is spilling from the rear into the front tank . Tto throw another loop . I had quit using the A/C because of intermittent cooling . When I used again after not using it for awhile , it seemed like the front tank again filled , after not filling for a long time . What do you guy' s think ? Tthe truck still runs great .Id like to use both tanks again . I do have a little problem starting , but not a big concern right now.

 

Sounds like the check valve in the front tank is bad. It needs to be operating properly even if you're only using the other tank. Otherwise it will just let fuel run into the front tank whenever the system is pressurized, and you won't be getting proper pressure to the injectors either.
You need a new front pump is my thoughts on that one.

 

Same thing happened to my truck a few years back. Front tank had a hole in it so I only used the rear. Driving back to base from Tampa and I had to pull over every exit and siphon gas from the front tank back to the rear...it was a long night. Like Chris said, bad check valve. I replaced the whole tank/pump/sending unit while I was at it. Now I have two working tanks!