Ya.. you'd really be suprised HOW STRICT the gov't is on this a/c shit... before i even touched my first a/c repair job at my toyota dealership, i was fronting my Federal and State Certifications to do so...
... We here this all the time.. "can you top off my a/c system.." or.. " i know its leaking, i just want it to last through the summer."
... and i take particular joy in telling them "absolutely not" and then going through the list of reasons why i will not...
Chrisco is very correct...
You have to "evacuate" the a/c system, clearing it of any remaining refrigerant... the system could blow HOT air and still have R-12 or R-134a in it..
then you draw a vacuum on it, usually if it holds -28, -29 for 10 min its good. If there is a small leak, you'd draw a vacuum, and inject dye.. a large leak.. well... look for it...
Then you add oil and charge the system.. the whole process is very clean, its meant to not release any refrigerant into the atmosphere..
Actually.. a little fun fact... ( someone correct me if i'm wrong) When R-12 or freon is mixed with air, it becomes combustable, and when combusted, one of the byproducts is phosgene -- a chemical weapon used in WWI that attacks the tissues in the lungs similar to mustard gas
ya-- i was right.. "One hazard of R-12 as a refrigerant is the health risk should leakage of the vapor come into contact with an open flame of high temperature (about 1022°F) and be decomposed into phosgene gas, which is highly toxic"
http://www.tpub.com/content/construction/14279/css/14279_206.htm