The OP wanted to know if the fuel lines could form a "loop" that was sealed. He secified the Dubro cap.

The Dubro cap vent itself doesn't have anything more than a metal tube stub (on the left) that requires a short piece of fuel line to plug the "probe" into. On the right is the fill fitting that has provisions for a tube on both ends, one to go to the clunk/filter in the jug, the other to the fuel fill probe. I'm not aware of any fuel probe that will seal when plugged into the metal the vent tube itself.




However, just about any "probe" whether it has an "O" ring or not will seal when plugged into a piece of fuel line tubing. Loop the fill line/probe to the vent line & you have a sealed "loop" just as the Op asked about. The Slimline Excel probe, even though it is much larger than the typical fuel probe used W/standard fuel line filling will still seal when the vent tubing stretches over the end.



My set-up is actually the Dubro fittings used W/the inner portion of a standard fuel jug cap.