Quote Originally Posted by PaulG View Post
I think it's the unloader valve I was referring to above Joe. It's a bigger brass unit with a spring/diaphragm inside ..
It could be the unloader valve or it could be the pressure switch.
If unloader valve it will have a (air) connection to compressor head or between compressor and tank (there is usually a non return valve where Pipe from compressor enters tank).
If Pressure switch, there will be a (air) connection to the tank (sometimes in at same point/in conjunction with tank pressure gauge).
Hope this info is of use.

Joe