They start leaking out the exhaust. I would not attempt to rebuild the one you have yourself. If no one has monkeyed with it, it should have a tag. If there is no tag, I can probably identify it if you will post a picture. Or take a look at this Bendix catalog.

http://www.bepco.biz/Catalogs/AirBrakeCat/80-82.pdf

The R12 valve is a popular valve that is still used today.

If you look at the R8 valve, it shows a picture of of the bottom side. The round piece of rubber in the middle with what looks like a screw in the middle is the exhaust.