I know a lot of us depend on compressors in our gun building activities, so do I, I have a 65 gallon upright that I got in a trade a few years ago and wanted to make sure it drained so it didn't incur rust damage to the tank, so I hop down to harbor freight last time I was in a bigger town and pick up one of their cheap compressor auto drain valves and install it, worked great for about 6 months!

Then or course being cheap it started leaking!!!  Normal for cheap fixes, thought crap can't do all my chores without my compressor!  take the valve out and torn it apart, and found out what goes wrong in these things, seems a lot of people have the same problem with them and either buy a new kit, or junk it and back to the old way of draining them.

Well what goes wrong, is there is a 1" PVC diaphragm disk that the air pushes a plunger up against and opens and closes when it turns off to drain the water, the disk ends up tearing where the nut seals to it and you have a leak, so I look around in my local town and can't find anything to replace it for less than $20 for a small sheet of rubber!

Crap, the valve only cost $10 new!  So I am digging around the tool box and find an old inner tube patch kit, notice their is a rubber patch in there just about the same size as the diaphragm that tore, trim it down a little to match the PVC disk, put it all together and it works just as it should again!

$1.99 for the patch kit and it comes with two of the rubber disks that are the right sized for fixing this.

So that is my cheap fix solution for a cheap part for a important part of our home workshops!

... what exactly do people use air compressors for when working on guns?

I use mine for blowing chips away from it when I am machining or using my lathe to make parts for a gun