I do commercial refrigeration for a living. the same principle applies to air conditioning. DO NOT LET AIR INTO THE SYSTEM, IF YOU DO HAVE IT PROPERLY EVACUATED AS AIR BECOMES MOISTURE AND MOISTURE BECOMES ACID AND ACID DESTROYS COMPRESSORS, PLUGS DRIERS AND IS HARD TO CLEAN UP AFTER ALL THIS HAS HAPPENED. IT IS BEST TO EVACUATE AT THE TIME THE SYSTEM HAS BEEN OPENED.