Did you see my picture? I just use this clear O-ring and that makes it possible to get a seal. Probably I cut it off from a sleeve or something sex toy related, it was longer time ago.
I don't need to take off the valve. Looks like pedud did. I had difficult time a while ago, as the idea was to fit the damn rubber cork over the valve. Nope, no luck. But with this O-ring type setup, really easy.
The valve can be removed, should come off easily with a screwdriver or similar tool.