69 Dodge Dart Venting

^^Others are correct.^^ That cap should be ABSOLUTELY sealed. If it's leaking, it's because the gasket, or cap, or fuel neck is damaged. The vent is simply an open ended tube, ends inside the frame rail.

This is right out of a factory service manual. You can download them FREE from MyMopar

http://www.mymopar.com/index.php?pid=31