i picked a used craftsman 1.5 hp. 15gal compressor in pieces, for $10. I put all the pieces back together, and its working, though the regulator is a bit wonky ( i had to free up the adjustment screw, took me 2x so far but i think i got it) out put flow was restricted before that.
Im assuming the motor should shut off once it reaches a certain pressure, its rated at 150 psi. does it have to get to 150psi before the motor "should " shut off? Ive let it charge to 90 psi. i should probably just run it for a bit more and see how it goes. But i wanted to know how the pressure switch works, and even dialing down the reg, didnt put it into shut off mode, so i was a bit nervous about letting the pressure get to high.
believe it or not, this is my first compressor that doesn't use a cigarette lighter socket for power, so im still learning ...
In reply to 924guy:
My Craftsman compressor automatically shuts off above a certain pressure. It also starts back up automatically when the pressure drops past a predetermined point. With a used compressor that you had to reassemble, be sure to check the safety release valve's condition and examine the tank for rust or dings. Any weakness in the old tank could cause a life threatening tank explosion. After each use of your compressor, use the drain plug to lower the tank pressure. This reduces stress on the tank when not in use and drains any corrosion causing water.
Another maintenance item is replacing the inlet air filter. It's usually a piece of small black foam. Old ones can deteriorate and fall into the compressor.
Enjoy your compressor. I use mine on a regular basis.
The regulator doesn't tell the compressor motor when to shut off. It actually has nothing to do with tank pressure. As Cagle said, your compressor will turn off automatically at a predetermined point (150 psi) and turn on again at another (125 psi??). The regulator just adjusts the air pressure that goes through the air lines into your tools.
Example: if you have an impact wrench that is rated for 60 psi, you adjust the regulator until it sets at 60 psi. You may have to run the tool for a second to make sure it levels out there. Sometimes you have to fiddle with it a time or two to make it sit at your tool's setting. The tank's pressure will still be 150 psi.
Also, follow Cagle's advice on draining the moisture from the tank. This can cause a pretty serious failure that could injure people. My dad's compressor exploded when we were on vacation once although it was the compressor assembly itself and not the tank. It was 4 years old. I found the connecting rod and some other bits, but the piston must have just shattered because it was nowhere to be found.
Edit: Oh yeah, turn it off when you're not using it. 
They leak through the fittings and hoses a little bit and will run when you're not around.
exactly what i needed to know, thanks all!
The compressor is like new, i dont think its more than a couple of years old and doesnt appear to have been used much. no dents, or even scratches for that matter. From what i see, they just jammed up the cheap regulator adjustment nut, and failed to identify the problem after taking the power head off (or gave up.) no sign of rust , and just just a bit of (clear)water spray when i drained it.
im going to let it charge fully and see what happens, ill bunker up the first time :)
Typical pressure switch settings are 90-110 psi for smaller units like that. You can usually set the differential in the switch, and 15-20 psi works OK.
The blow off valve or whatever you call it, can be reset.
It would be wise to let it get 50 psi and pull it, to make sure it works OK.
