Air Compressor puzzle.

[the evil twin]

As others have said... usually a symptom of a faulty switch or blocked pressure port due to corrosion

Assuming the PRV and Pressure Switch are not integrated
1 - remove pressure switch (leave wiring connected), run compressor if tank is empty, either way air should freely escape switch port - if not port is blocked
2 - remove wire from switch, compressor should stop - if not wire is low resistance to earth (pressure switch should be on earth side of comp not 12V)