This pressure switch is used to turn the air compressor on when tank pressure is below 135 psi and will turn it off at 150.   It has two spades on it and is commonly used in our RidePro Analog Gauge Compressor Systems.   (PRE1500)

How to determine if your switch is bad?   
Symptom - Compressor will not turn on:   Verify that tank pressure is below 135psi, then use a jumper wire to connect the two spades.   If the compressor then turns on the switch needs replaced.    

Symptom - Compressor will not turn off:   Verify that tank pressure is above 150psi, then disconnect one of the wires to the pressure swtich.  If the compressor then turns off, the switch needs replaced.