My air compressor regulator has stopped holding pressure correctly; the primary symptom is that the regulator leaks and the tank never reaches the cut-off pressure of 150+ PSI and instead leaks at whatever pressure the regulator is set to.
Let's take it apart and see if we can figure out what went wrong:
Here's the core of the regulator and we can see the spring is compressed and the piston cannot move. Odds are good this is the problem:
Let's gently open that up and see what we see inside; looks like there's some corrosion in there:
Did that fix it? Yes! The tank now holds more pressure than the output line of the regulator as you can see on these gauges. The left one shows Tank Pressure and the right one shows Output Pressure (150 PSI versus ~90 PSI)
A bit of elbow grease and some machine lubricant and the corrosion has been removed. The mechanism now moves smoothly and the fitting is ready to be put back in: