It's your back flow line. If you look under your truck you will see a secondary line going to the main fill. Some call this the vapor (for us truckers who deliver gas to gas stations). When I use to work for Dejana and we did Ford body modifications if the vapor wasn't set at a right angle we would get guys coming back, having the pump shut off after a few gallons. A few tip ties putting the vapor line at a aggressive level always helped.