Some decades back I made bourdon tube gauges for bikes. I never used a valve, so you had to read it while on the bike. I only stopped when my vendor who had been making gauges in PA for over 100 years, moved production of the most generic gauges to the orient (first Taiwan, later China).

The OPs gauge was made by or for Meiser in Chicago. Odds are the check valve in the OP's gauge is frozen open, or dirty so it doesn't seal. You can unscrew the entire assembly form the body of the gauge and take that apart (it's a simple mechanism) and probably fix the valve. Otherwise you might see if Meiser will sell you a replacement for the valve assembly.

BTW- Gyozadude is right, gauges are most acurate at the central third of the scale, ie, from 35-65 on a 0-100psi gauge, with fall off towards the ends. Also repeated use to the top 5% of the scale can fatigue the tube, leading the gauge to read high permanently. Bike tire gauges should be 0-160psi, or 0-200psi.