The copper line on the side of the pressure switch was never hooked up. I Tee in to where the gauge comes out of the tank and ran a copper line down to the switch. The pressure builds to the high setting then bleeds off to the low setting and starts up again and repeats. The air bleeds out of the copper line that I hooked up to the pressure switch. This is a used compressor I picked up from a private seller and the copper tube was never hooked into the tank from the pressure switch. I disconnected the tube and it shuts on and off where it should. Can I keep running it without the tube to the switch? The pressure guage and over-relief valve are good. Could it be the upload valve from the low pressure side to the high pressure side adjustment?
That little copper line is an unloading line, to bleed compressed air out of the compressor head so that the motor can more easily start the compressor. Pressor switch should be threadded into top of air tank, and the unloading valve is built into the side of the switch. When the tank reaches the high pressure setting the switch disconnects power to the motor and opens the unloading valve at the same time. The unloading valve should be connected to the compressor discharge line through a check valve with an unloading valve port upstream of the check valve. These valves are usually threaded into the top of the air tank as well. Sounds to me like your valve was leaking and someone bypassed it.
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