Help me fix my air compressor v. what's missing?

[Heckraiser]

I got this air compressor for free with my house and all the previous owner would tell me is "it doesn't work." After sitting in the garage for almost 3 years, I finally got around to working on it. I filled it with oil, bought some wire and wired it up to the 220v plug and to my surprise, it fired right up! It even builds air pressure if I close the output valve and put my finger on the other opening.

That opening is my concern though. I know something should go here, but I don't know what. I also don't know what causes the compressor to shut off automatically when it reaches the desired pressure. Can someone help me figure out what else I need in order to safely run this thing?

Here's a picture of the "missing" spot. I'd be happy to take photos of other parts of the compressor if it would help.

Thanks!

 

[e9999]

well, if there isn't another one someplace else, I would put a safety valve in there. I did put 2 on mine just in case...
The switch looks connected to the tank and likely reads the pressure, if it's like mine it's mechanically-actuated and adjustable. It's a clever if somewhat crude spring-loaded flipping device.
Looks like it's also missing the casing around what looks like a fan. You may want one of those on to help with cooling and prevent folks from getting their fingers chopped off. And a guard over the belt while you're at it. That is if you don't have those already.

 

[Heckraiser]

So after poking around it some more, here is my theory on how the pressure switch works. It's small air line with what looks like a valve stem sticking off the end of it and when the pressure builds high enough for the end of that stem to contact the metal tab that's right in front of it, it bleeds off excess air pressure. Is that even remotely correct?

If so, how do I know what PSI my switch is set to? I would like to put a safety relief valve in that hole, but it would need to be a higher rating than the switch, right?

 

[Pin_Head]

The small air line on the pressure switch is an "unloader", and when it turns off the compressor, it vents the air line between the compressor and the check valve so that the compressor does not have to start against a pressure head.

Some pressure switches are ajustable within a range by turning a screw inside.

 

[Blue77FJ40]

So, apparently, it didn't work because it wasn't properly wired and powered.

That's how I got a great floor drill press for $75 at an estate sale. The power switch was busted. New switch was $12.