Originally Posted by
Ernie Misner
What brand is your fuel cap fittings? The DuBro and Sullivan/Slimline types have a small o-ring on the probe that seals very well when you put it back into place in the cap. No need to remove or put the stock cap back on the jug at all.
The OP linked the Dubro cap on Tower's site.
No need for an "O" ring, Just push the fill line over the brass vent tube stub.