This could happen for any number of reasons, but many times this is caused either by the Check Valve in the inlet of the tank, or an electrical issue.
First, make sure the brass Check Valve in the tank is opening and closing like it should. There could be debris inside the valve which will cause this to happen.

You can try this test first, there's a video and a text doc on our site which will show you how to perform this simple test.
Just click here: www.mastertoolrepair.com/how-to-test-check-valve-p-22524.html

You may also want to take the cover off of the motor and make sure the piston moves up and down in the cylinder by hand, confirm that the conrod bearing is in good shape.

You can view much more technical trouble shooting docs on our site at the below link as well:
www.mastertoolrepair.com/air-compressor-...310_485_486_488.html

Thanks for the inquiry!
- MTR

You could be right about the Pressure Switch being the problem, certainly. It could be the copper contacts in the switch are not working probably, or there could be a short. You can probably test the switch with a volt meter, or have an electrical supply or motor repair shop test it for you.

Most likely the problem lies with the pressure switch or the motor since you already ruled out the other options.

You can check out some helpful trouble shooting documents on our website by clicking on the below link, which will possibly help you futher:
www.mastertoolrepair.com/air-compressor-...310_485_486_488.html