I spoke to Ridgid tech service, and they told me if it does not come off when you pull on it. Then it may be the style that has a label on it, that covers a screw that is holding it on to the regulator.
I have a Ridget OL50135 compressor that when I use it with a nail gun and fire one nail you must wait for approximately 30 seconds to a minute to fire again. Also in an attempt to inflate a tire, as soon as you place the nozzle on the valve, there is a short burst of air and the output gauge drops to 0. Remove it from the tire valve and it goes right back to normal. Both the tank gauge and output gauge show normal pressure. I disassembled and cleaned the regulator but it made no difference. I believe that it may be the 17398 pressure switch but I am not sure.
You will likely need new rings and a new cylinder. There will typically be where to both the piston and the cylinder. You will need to open up the compressor and inspect it. It there are any scratches on the cylinder wall it will definitely need to be replaced. At minimum a machine shop should hone the cylinder when new rings are installed.
The piston rings are part numbers; #17153, #17158 and #17163. The cylinder is part #17328. All of these parts are available from http://www.ereplacementparts.com.