grumpus

Inside the pressure gauge is a curved metal tube that distorts as pressure increases to deflect the needle - it's likely there's a crack in this that's allowing air to escape.
​​​​​​Pressure gauges have a standard pipe thread, so it should be easy to find a cheap replacement with similar pressure range once you've determined the thread size. Note that the American NPT pipe threads are different than the British BSP threads that most of the rest of the world uses, stuff sourced online may not state which system it uses just e.g. 1/8 inch or 1/4 inch (which also doesn't correspond to the external thread dimension).