Replacement pressure switch arrived and I fixed the air compressor.
Surprising amount of plumbing on it. All fittings matched except the small pipe on the bottom had to have its flange cut off as the new one needed an olive instead.
Also surprising, no air leak on the first try 
#DIY #Repair #Compressor #AirCompressor #Gas #Plumbing #UnderPressure
TIL!
I was wondering what the small extra pipe on the bottom of the pressure switch was for (with the olive). The big one is the one that pushes on the membrane against a spring and switches the contactors.
But there's a second, small diameter pipe that goes from the bottom of the switch to a three-way fitting where the braided hose from the pump enters the cylinder.
Not easy to find out, but inside that three-way fitting is a valve that depressurizes the air in the pump hose when the compressor switches off, which also explains why there is a small hiss everytime it switches off.
Why? So the pump motor has an easier time starting. Instead of pushing against the remaining pressure in the cylinder, it only gets atmospheric pressure.
Y⃒̸̷̝̜̙ͥͥͥngmar