The air leak would have to be in either a plumbing joint, drain valve, rust hole in the tank, the pressure gauge, the quick disconnect, or the air intake valve -- there is nothing else. You can either dunk it in a large container of water or mix a small amount of detergent in water and brush it on all possible leak areas -- just like you would do to find a leak in a tire. The quick disconnect is the most likely culprit and the pressure gauge is second on the list. The most time consuming and expensive to fix is the air intake valve which is probably a thin metal flapper. If the leak is really fast, it is very likely that the flapper is either cracked or has broken off completely, in which case it may have been swallowed into the cylinder. This would require a rebuild kit that consists of a cylinder sleeve, piston, intake and exhaust flapper valves, and gaskets. Many years ago, I paid about $50 for one of these kits, but I am sure that it is much more now.
