I think there's a trick to getting a reliable seal with swing tops: seat the cap by hand and compress the gasket with your thumb, then toggle the bail...
Excellent tip. In a way, it reminds me of the technique I read somewhere on HBT for sealing a corny keg: hold the cap up in place, let a little gas into the keg; it will seal the cap, at which point you lock the handle down. Thx for the post.