Valve in a valve?

[Cleggy]

Here's a tip - score all the way round the valve, cleanly in one joined up line - do so as close to the biscuit (the bit with the wire/pin feedthroughs) as possible. Then wipe the scored groove with WD40 and with the valve wrapped up in something protective and wearing eye protection & gloves, tap gently around the score with the end of a decent screwdriver. The "hot wire" (W or NiCh) technique is also very good for tubular work - lots of videos about this online.T

Doesn't need to be WD40, any liquid will do and most glassblowers use spit (i.e. they just touch it with the tip of their tongue). The reason why it works has to do with the "Griffith Crack Theory" of materials, and 40 years ago, I might have been a bit more informative, but alas I will have to direct you to Google now if you need more info .