Gas Tank --Fuel line Question

Ok I'm workin on a 68 coronet and heres the question, from the tank you have your main fuel supply to the eng, then you have 2 vent tubes off the other side, I know the one vent tube goes up & around through the trunk where does the other one go? From what I see it comes out & goes up to a hanger & stops there. The reason I ask is I seem to have fuel leaking from the vent tube that goes through the trunk. the other one has a piece of hose with a bolt in it because it seems like when I filled it up it would leak from that line. Now I did put a new tank & sending unit in and the new sending unit has a nipple probably for a return line can I run a piece of hose from that to the line that has the bolt in it? or is that just stupid... need your help here guys.. gas aint cheap

btt