car air con will always lose the refrigerant over time because of the vibrations throw the car, engine, etc.
if its left un-used throw winter the seals dry up and let buy, thats why they always tell you to run the air con in the winter at least once.
as forletting all the refrigerant out (gas) where the connectors go if you take the black caps off (they look like tyre valves) and depress the srader valve with a screwdriver this will let it go. also it would be better to do this on the smaller pipe as this is the high side of the system under more pressure you'll get a fluescent dye come out with it so don't be to alarmed.