1994 F150, 4.9, automatic, 114k miles, longbed... just had trouble filling my front tank on my truck today. Put the nozzle in and pumped gas in but nozzle shut off almost immediately when gas backed up in filler tube. Kept trying and could only get nozzle to run for a second or so each time. It acts like the tank is full but I know for sure that it is not. Later tried slowly pouring gas into tank from a gas can but had the same backup problem in the filler tube. Is there a vent or some other device that would keep the fuel from flowing into the tank? Any help would be appreciated. Thanks.