Figured out while putting the GN back together that I didn't have a boost gauge. I had sacrificed mine to the son's car, an Autometer carbon fiber one that came with the car.
I found the old, broken VDO in a parts pile, and took a close look at it. It had frozen at ~4PSI , it's a -30-0+25PSI vac/boost gauge. After taking out the light bulb, I could see the bourdon tube. After poking around I "unfroze" the needle, and it moved fine. Maybe the moisture did something, I don't know. Using some tools, I tweaked the bourdon tube until the gauge was at 0. I hooked up my Motormite vacuum pump, and the vacuum readings were spot on between the two.
I was curious about opening it to clean it, the front glass was fogged internally. After Googling a bit, it seems the old VW guys at TheSamba have experience taking apart the VDO gauges with the crimped bezels, mostly speedos..
Turns out all you need is a small flat headed screwdriver, some patience, and a few minutes to rest, as your hand WILL Cramp up.
You pry until you get the edge of the screwdriver under the crimped part, then work it around, lifting the crimp as you go. After going around a few times, the gauge will come apart. I got it apart w.o damaging the bezel at all - actually, now that it's out, I can try to fix a few dents it had from the years of use.
I plan to clean and lubricate the mech, and repaint the faded indicator needle. I won't be using this gauge - If all goes well, I will put it back in the T (The rest of the gauges are VDO) and save the Autometer as a backup.
Exploded view, with screwdriver used..
I found the black plastic lens holder is cracked, that doesn't help. I will glue/reseal this. I put the rubber gasket in the broken part to illustrate:
Pic of internal mech with flash:
Another angle w/o flash:
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