Hi Eric, I really appreciate having someone knowledgeable to ask this one. I love everything about my 2001 C3500 except the towing range. I'd like to add a smaller transfer tank (30 to 40 gallons) and wondered about the feasibility of feeding directly into the filler neck with a low pressure fuel pump, something in the 3 to 9 psi range and 30 to 45 gph feed. The vent tube comes up to about an inch or so from the neck and seems to be capped off. Is this a cap, or a pressure relief valve (doesn't look like one), and would I be letting myself in for trouble if I fed directly into this tube. Thanks loads, Jim.