the vacuum guages and probably the pressure stuff too is usually a curved hollow pipe that when pressurized (or de-pressurized) curls up and in turn, turns a small gear that turns the needle. I have a SW vacuum guage like that and it registered 4" vacuum at rest. I pulled the rack away from the needle pinion and jumped a few teeth and now it reads 0. pulled 10" with my old gauged mighty-vac and it registered 10 so I think they may be 'workable' if you take the face off, and maybe 2 phillips screws in the back. I recall you can manually bend the little tube also to zero the gauge if jumping a tooth is not practical.
