When you have a new system the first thing you do is hook up a vacuum pump, you pull the system down for about 45 minutes, this creates negative pressure AKA vacuum. It also removes any condensation from the system, once you hook up the can it sucks it in because you have a vacuum AKA lower atmospheric bar level. if the system has a leak and the pressure is allowed to go out the leak you end up with a system that won't take freon. You gotta find the leak first bro, fix it, pull a vacuum and re-charge otherwise you are throwing away what? 10 to 13 dollars a can of freon for nothing!
