Thanks for the post!

Firstly, what is the model# of the compressor? If it's a Sears then it will start with either a "919" or a "106" most likely, then have 6 digits after that, e.g. "919.166575".

It sounds like your Pressure Switch is defective. This is supposed to automatically cut the unit off when it reaches the pre-set pressure (usually 125 PSI for a single-stage unit). If the Gauges are accurate and your ARE reaching 125 PSI and it's still not shutting off, I'd try cleaning the copper contacts inside the pressure switch, or replacing the pressure switch.

Secondly, make sure your Check Valve in the tank is holding pressure in the tank and not back-feeding air to the pump head. You can test this easily by manually shutting it off then listen for leaks around the pressure switch area. The tank should hold pressure and not bleed it out....if so you have a faulty check valve.

Let us know what you find, we'll get to the bottom of it.

- MTR