Or it could be even simpler, and have a nicer fix. Often gauges have a piece in the inlet to limit any effects of varying pressure. I doubt any manufacturer would bother to remove that even if it wasn't necessary. Sometimes a tiny piece of debris will pass through the tiny orifice, but get jammed on the way out trapping some air. Remove the guage, remove the snubber which usually comes out with a really small screwdriver, and flush everything out with your favorite cleaning fluid -Water is usually fine for an air guage, allow some time for anything burnable like alchohol to evaporate, and reassemble.
If it is trapped debris the guage will return to normal as soon as the orifice is removed.
If it's been spiked then the stuff David recommends is the easy fix. Very few gauges are readily repairable internally, and the ones that can be repaired are just too expensive for a bike pump. If you can get at the internals, figuring out how to realign the needle with a minimum of bending can be a fun project for a rainy day.
The training I got at the industrial repair was being given a box of broken gauges during a slow time and told to see what I could do with them. Any I could fix got used for stuff around the shop that wasn't critical.
Steve Birmingham Lowell, Massachusetts USA
Message: 16 Date: Fri, 26 Mar 2010 00:30:19 -0700 From: "David Snyder" <dddd@pacbell.net> Subject: Re: [CR] all pumped out (repair) To: <classicrendezvous@bikelist.org> Message-ID: <F13B6FA84B4F4ECAB22AF629302E8BAD@ddddPC> Content-Type: text/plain; format=flowed; charset="iso-8859-1"; reply-type=original
It sounds like someone made a firm stroke of the pump against a presta valve
that momentarily refused to open, stretching the coiled tube that actuates the geared needle as the pressure spiked over ~200psi.
I've fixed these by manually twisting the needle backward. Note that forcing the needle has a ratchet effect in one direction, and may skip a tooth in the gear mech. This will create a gross readjustment to the needle's rest position, necessitating some follow-up twisting/bending of the
needle to a finite pressure/position verified by a guage (i.e., the gears will slip under force in only one direction). It's not usually accurate to zero the guage, since zero usually has the needle pre-loaded in the negative
direction against a stop pin.
If you can twist the needle on the shaft or remove the needle and push it back on in a more proper position, it may be easiest/best. You'll also have to lift/bend the needle a bit to go backward past the stop pin and then back
again after resetting. In most cases, the final tweak toward calibrated accuracy is easiest to achieve by bending the needle itself. Note that a full turn or so may be needed to reset the needle back even a few degrees, but this seems to cause no harm.
I have re-set many geared, bourdon-tube guages this way, and none was the worse for it. The important thing though is to have a usable standard (guage) for establishing reasonable accuracy.
David Snyder
Auburn, CA usa