I examined my fittings and took some pictures. The key element is the o-ring seal in the vent hole that seals to the fuel fitting when not in use. One of mine was shot. Next, as most of us know, the plastic nut on the DuBro "Fuel Station" is always coming loose. There is a big O-ring under it and it may well not be sealing when the nut is loose. I air tested my three fueler cap setups and none of them was really sealed. I am going to try to get one of those Slimline caps that you have, because then there is no question about being sealed. However it is $30 for the #2528 cap. My LHS says he can't get Slimline products anymore, but he has one of the #2013 which is the "360" cap with a mount for a hand pump, $40.