Hi John:
I had the same problem with an altimeter, new out of the box, but I had it in storage for eight years. Off to the shop where I learned that this is a common problem. The sealant around the glass cracks. This is the most common cause of a flunked static system test.
Solution: clear nail polish around the perimeter of the glass. Wicks into the crack, but is reasonably tough and flexible, works fine. Expect to reapply every few years, particularly if the cockpit is thermally cycled a lot due to bigger day/night temperature swings wherever the airplane is stored.
F