62 LANCER GAS LEAK

I recently acquired a 62 Lancer. Not much history on it. When i fill it up all the way, it leaks fuel out of the top. I haven't gotten in there to check it out yet, but was wondering if anyone had some advice. The unit coming out of the tank looks like it's been replaced at some point. The cap says not vented. Not sure if that matters, but I wanted to throw that out there.