A word of caution here for those of us thinking about working on a/c:
-Never, ever try to add refregerant to the high side, ONLY the low side. The high side pressures get high enough to blow up the refrigerant can. Think what that might do to your hands and face.
-If you don't know which side is the high side and which is the low, they leave the repair to a professional.
-Refrigerant is to be added in gas form only. Do not turn the can upside down when you are adding the gas, which would cause liquid refrigerant to enter the system.
-Never try to use any heating element or torch, etc. to heat the can to get the refrigerant out. I read a case in lawschool where a mechanic used one of those old beverage heating elements to heat the water the refrigerant cans were in. He picked a can up and it blew up, taking his hand off.
-Wear safety goggles.
-Don't breath the gas if for some reason some comes out of the can or system in your presence.