Pressure Switch issues can take a couple of forms.
There are real pressure switch problems, things that are actually wrong with a pressure switch, or there are problems that appear to be pressure switch related, but are not. This page deals with both.
Some pressure switches have an integral unloader valve (inside the cover) and when air is bleeding out of the bottom of the pressure switch, it may actually be flowing out through the unloader valve inside the pressure switch. Other types of pressures switches have an unloader valve installed adjacent to the switch. If either type of unloader has air bleeding out of it, it could feel as though the pressure switch was leaking air.
What they are experiencing is not likely a pressure switch or an unloader valve leak at all. It is a problem relating to the check valve.
The unloader valve is a needle valve which is linked to, and responds to the pressure switch as the switch reacts to the compressor tank pressure. The mechanical action which opens the contacts inside the switch to shut off the compressor motor, is linked too, and also opens, the unloader valve. This unloader valve then connects the compressed air trapped over the compressor piston to atmosphere, and allows that trapped air to escape, unloading the compressor.
You can hear this happen. When a compressor reaches the high pressure set point and stops, there is the characteristic "Psssssst" sound, the sound of air escaping. This is the sound of the compressor pump being unloaded. And it should be a brief burst of air, normally less than a second or two.
Between the unloader valve and the air in the tank there is a check valve, or as it is also known, a one-way valve installed.
Typically, this check valve is installed where the line from the pump head reaches the tank and is, too, sometimes a part of the fitting to which the line from the pump is attached.
Upstream from the check valve on the line from the pump head, there will also be a tee. This tee may, too, be part of that same fitting. This tee connects an air line that is plumbed over to the pressure switch. The air in this line is at pump pressure, and it is the air pressure in this line that the pressure switch reads to know when to turn on or turn off the compressor motor.
This line is also the pathway of the air from over the compressor pump when the motor stops and the unloader valve is opened by the pressure switch linkage.
Over time, the tank check valve can wear or get airborne debris deposited on its seats or seals. As a result, instead of being closed tightly by the air pressure in the tank, compressed air bleeds by the check valve.
When the compressor stops, the unloader valve is opened automatically. Now, though, instead of just dumping the air trapped over the piston, the open unloader valve allows the tank air that is leaking past the failed check valve, to flow out to atmosphere.
Remove the check valve (after pulling the plug on the compressor and exhausting compressed air from the tank) and clean it up, or if it badly worn, replace it.
That should solve that problem.
Unplug the air compressor and please do not use it again until such time as you have replaced the switch. This is a major safety issue and must be corrected immediately.
If the compressor keeps running and the air pressure in the tank stops rising, the compressor cannot shut off as the tank pressure will never reach the cut out pressure setting. Your air compressor will ulitmately overheat or run itself to death.
This is not a pressure switch problem. This is likely a problem with air flow into the compressor, from the pump into the tank, within the pump itself, at the check valve, or you have a leaky check valve. Here are reasons why an air compressor will not build pressure, but keep on running.
You are using an air tool, the pressure in the tank falls way below the normal compressor cut in pressure level, but the compressor ultimately drains completely and does not start at all.
Unplug the compressor and have a look under the pressure switch cover for any obvious problems. Is it filthy, or burnt, for example?
If dirty, clean it up (with the power off), re-cover the switch, and try again. Maybe even give it a gentle tap with a screwdriver handle.
If the compressor still won't start, the plug is in, the pressure in the tank is below the "kick" in pressure (or at zero PSI gauge) then it may be the pressure switch that has failed.
If you are comfortable doing so, use a multi-meter to check for power flow from the pressure switch to the motor. If the pressure in the tank is below the normal cut in pressure level, the pressure switch should have tripped, and power should be flowing through the switch to the motor.
If this is not happening, then I suggest you replace the pressure switch.