For many of us compressor users, the unloader valve is, if not integral to the pressure switch, located beside it and activated by it, which is why I include it on this page.
In the photo left, you can see the unloader valve on the side of the pressure switch.
The unloader valve is toggled by an appendage protruding from the side of the switch. That appendage moves up and down as the pressure switch trips on and off, and in so doing, operates the valve actuator by depressing or releasing the pin, seen protruding down from the valve.
If the pressure in the tank is below the normal cut in, then plug the compressor in, and use a multi-meter to check and see if voltage is flowing through the switch to the motor. If there is no pressure in the tank, the switch should have tripped to ON, and power should be flowing. If it is not, then perhaps the pressure switch is pooched.
There are a couple of issues with pressure switches that are common problems.
One of those problems is often confused as a pressure switch or unloader valve leak. In the event your pressure switch or unloader valve is bleeding air constantly, the problem may not be either. The compressor check valve may be the issue. Read about that issue here.
And, if you are still having trouble with your compressor pressure switch, here is a pressure switch troubleshooting page that has lots of postings from folks that have a compressor switch problem, a lot of advice you can read and comment on, and a place for you to post your own question of the answers there do not suffice.