Gas tank vent line puking fuel

[wilkin250r]

I have a 72 with two lines coming from the fuel tank, routed into the engine bay. One is routed to the fuel pump, the other is just hanging out with some sort of breather cap on it. I am assuming it is a fuel vent line (perhaps that assumption is my first mistake).

Problem is, if I fill up the fuel tank beyond about 3/4 full, fuel will begin to flow out of that vent line. Coincidentally, the end of this vent line is mounted at a height I'd assume to be right around 3/4 height of the tank. SO it would appear that as soon as the fuel rises above the end of this vent line, it becomes a drain.

I'm sure that's not normal. The knee-jerk reaction would be "just mount the breather end higher than the tank. Problem solved!" That's not a solution, that's a band-aid. I want to actually SOLVE the problem.

So educate me. Those that have vent lines on their tanks, where are they routed to? Or am I mistaken and this ISN'T a vent line at all? If it's not, then what the heck is it?