Waxed dental floss, the heavier type, will also do the trick. Flavoured or non-flavoured is up to you.
djm
It looks like the sort of valve Dennis Havlena describes here.
That’s it exactly–I was wondering why the top surface of the rubber looked wavy, as if it had been rolled.
Joseph, I’d also wondered about the gap you point out, but when I reseated the rubber, I made sure that the copper strip pushed the flap closed here.