Understanding how the pressure switch & check valve on my air compressor work together and why my pressure switch may be leaking air after shut off

When the pressure switch turns off the air compressor, it opens # 2 (pressure switch unloader valve) and the check valve (# 5) closes. When the check valve closes, it shuts off the air from the tank so that the unloader valve (# 2) can relieve all pressure above the check valve.

If air is leaking out of the unloader valve (#2) continuously after the compressor shuts off and then stops leaking when it turns back on then that means the check valve (# 5) is stuck open and not holding the air pressure in the tank. In this case, the check valve (# 5) needs to be cleaned or replaced.

The tank check valve (# 5) has a Teflon disc with a spring behind it that allows the disc to close up against the seat from the spring pressure when the compressor stops running. If any debris gets caught inside of the check valve and the disc can’t return to it’s seat and close properly, then the air travels up into the pump and out of the unloader valve (# 2). This is a very common problem that is often misdiagnosed as a faulty pressure switch or unloader valve when it is usually the check valve that is the problem. When everything is working properly, the pressure switch unloader valve (# 2) should release pressure for just a few seconds and then stop before the compressor is ready to start back up again.