Gas cap issue - now I have a fuel leak

[Coach]

I replaced my gas cap last week, the rubber was starting to tear apart and would no longer tighten. I went to Napa and got a replacement gas cap, different style, but the guy at the counter who has helped me in the past said the outside diameter was not important, as long as the inside diameter was correct (1 1/2 inches). Well, just today I noticed a small gas leak coming from the rubber hose that is connected to the fuel sending unit. It never leaked a drop in the 3 years I have had the truck. I tightened up the hose clamp a bit, slowed the leak, but not completely. I unscrewed the gas cap and it released a bunch of air pressure. Since then, the leak has stopped, but I have checked it a couple more times and more air pressure. I have a feeling the gas cap is not venting, since it is smaller outside diameter and not completely covering the filler neck flange, like the old cap, which is 3 3/4 inches. Could this be the problem with the new fuel leak??