I have three Bauer air storage cylinders (rated to 4500 pounds) and yesterday I noticed that one of the cylinder's valves is now freely turning (it won't turn on or off). I can isolate the cylinder, but I am not sure if the bottle is empty or full (If I had to guess, I would say that it is pressurized). The cylinder is fairly new (less than five years old) so I would like to replace the valve and not the entire cylinder. How do I bleed off the pressure safely so that I can replace the valve? Any suggestions would be greatly appreciated.