Mine did the same thing - now go back and look closely at the problem, I think you may find its a little different than it appears.
There are two little one-way valves which are part of the evaporative control system - right back by the filler hose. It appeared that one of mine was leaking, but instead, what had happened was there was a hairline crack in the filler vent hose (the approx 1/2" one) and it was leaking directly onto the valve and making it very clean compared to its surroudings.
I went and got a two-way barbed hose repair fitting in brass (from home depot - in the plumbing dept) and mended the filler vent hose.
If it is one of those little valves, I suspect you might be able to swap them out in place assuming you can get the age-stiffened line to let go.