It doesn't leak if it's new. It doesn't leak if you maintain it. It doesn't leak if you fix one with a little wax on the control rod on one that wasn't maintained.
In short, this is a non-problem. It's only a problem if someone's glued the baffle in place because they couldn't figure out how to light a candle and drip a little wax on the control rod to lubricate and seal it.
More to the point, even if they seep (doesn't pass a "pop" test but doesn't audibly hiss) it doesn't affect the playability or sound of the piece.
I've played on them for 40 years. It's certainly not a classical piece. As you adjust them they range in tone color from "very centered but not particularly bright" to "able to strip paint and cut concrete".
They changed the baffle, lay, and chamber designs over the years and the parts do not interchange from the early days to now. The early ones (pre-1980-ish) are rounder, fuller, and quite a bit more versatile than what you get from Zinner today. That's not to say they're better, just that they are generally brighter now than they were then.
40 years ago when I was the lead alto in my HS jazz band, it was a perfect piece for that, however, today I play in a small group doing latin jazz and I think the 2010 piece I got from Zinner is too bright for that application regardless of setting. As a result, I picked up a Meyer 7M copy from Kessler that works much better in taming the alto for this application.
Curiously, the tenor piece I've had since 1972 fits in quite well for this, and a pre-1980 bari piece I picked up has a wide range of tonal color that works quite well either in a big band setting or in a small group.
Tom Scott and Nick Brignola I'm not, but you can hear the tenor and bari pieces here if you're curious.
www.soundcloud.com/maddenma
On Blue Bossa I have the slider on the bari piece in full bright and you can hear all the harmonics it generates. Frankly, I find the harmonics make the piece a little tough to control on the bari piece with the slider set to full bright, so I don't generally play it there but usually push it back a notch or two.
On Black Orpheus I have the bari piece set to its darkest position, and you'll notice it is quite behaved, but nothing resembling a "legit" sound.
On Agua De Beber I have the tenor piece set about half way between bright and dark and you'll notice the tone is very centered but not paint-peeling. This is also usually where I play this piece, but I do adjust it as well.
Altissimo quite a bit easier when these are set full bright, but control tends to be a little tougher as well. A good comprimise is one or two notches back which reduces the height of the baffle but also reduces chamber volume -- which is why these just aren't suited to legit music and can be a problem on vintage horns with regard to intonation.
I haven't tried the Zinner versions (post-1985 or so) of the tenor and bari pieces, but my presumption is that they'll behave as they did with the alto. Quite a bit brighter on the new pieces than the earlier pieces.
That's what I know. Hope it helps.