Well, the sans lathe techniques would be entirely different for metal than what I would imagine for glass.

The valve piston is probably a standard size glass rod. Perhaps the valve body is blown around a suitably sized metal former to produce a bore that's close to the OD of the valve piston. All glass (body and piston) hypodermic syringes are common (I have several) so a good seal is certainly possible.

Another possibility is heating and pulling a glass cylinder until it's the right OD and then cutting a section out of it. Of course, the reverse of the procedure is possible - standard size tubing and heat-pull rod to make a piston to fit.

These are only guesses on my part, though. My total experience with practical glass blowing was making an ionization chamber for a physics lab in college. It wasn't near the complexity of this and, of course, had no moving parts.