[The Art Doctor]

Mine does the same. My tank is new Drake too and needs a vented cap due to a modern sending unit. Been trying to figure out where its coming from and I think it is between the seal and the top flat part of the neck as its slightly wet there when I take the cap off. There is always a small puddle between the slosh valve and the opening so I know the slosh valve is not terribly effective but probably it would be worse if it were not there. The cap is very tight on and off so I can imagine adding more gasket thickness would not help, possibly a fresh one as you are doing would be the trick. I'll wait and see how yours turns out.