The first thing to determine is whether the leak occurs while the unit is running or when it is off. The only times that air should leak from the pressure switch is when the compressor shuts off at maximum pressure or when the pressure switch is turned off manually with the on/off knob while the unit is running. The leaking air is actually coming from the unloader valve that is connected to the pressure switch. The unloader valve will have a black flexible tube coming off of it that connects to the check valve, which is located at the end of the copper tube that runs from the compressor head to the tank.
This air leaking would be a short hissing sound that lasts a few seconds and then stops. If the air continues to leak after a few seconds, this would indicate that the check valve is not working properly and should be repaired or replaced. If the check valve is bad, the compressor will not be able to restart when the unit reaches the cut-in pressure when the pressure switch is left in the on/auto mode. If the air is leaking from the unloader valve while the compressor is running, the unloader valve is bad and will need to be replaced. The complete pressure switch will not need to be replaced, only the unloader valve. The compressor will be unable to restart if the unloader valve is bad.