If it is all brass and glass, then a couple of drops of vinegar is where I'd start. Then after an hour or so of that treatment, rinse it out and then dry it out with a light dose of direct heat from a fine flame. No need to get it real hot, just hot enough to where you can't quit touch it. While it is still hot, or at least quite warm, apply a few drops of some good penetrant like kerosene, diesel oil, Tap-Magic, etc. and let the valve cooling down draw it in. Then let it sit for hours, days, weeks as you may choose. A few solid bumps with a soft faced hammer during the wait time may not go amiss. If you have an electric pencil, then several applications of that harsh vibration transmitted through some fairly hard intermediate material, such that the valve stem is not marked but the vibrations are transmitted. Finally a careful gripping with some soft jawed pliers, split leather in the jaws of some regular pliers is what I often use, and gently rocking the valve stem back and forth, alternating with some in and out pressure might eventually free it up. If not, then repeat the whole process again, or any part of it you feel you may have skimped on.
Hope any of this helps, and .... Good Luck!