Originally Posted by bvalva
Wilsoch, it sounds like the cap you are talking about covers the Schroeder valve (like those used in the valve stems of your car's tires) that are used to test pressure and add or remove the freon.
I have found more than a few of these valves left uncapped and they were the source of long term slow leaks.
So it has been off for a few months? There is a chance that the system has lost some of its freon charge. Get someone to check the freon charge. It if is low enough, in some systems only by a few ounces, you will not get any cooling.