Also look for oil on the outside of the air conditioning tubing or fittings. Oil travels with the refrigerant and can show a leak (Freon brand R-12 refrigerant has been out of use for many years.) This is not a DIY job. As mentioned above the existing refrigerant must be removed from the system with a recovery machine, repairs made, air evacuated with a vacuum pump, and the correct weight of refrigerant put back. The sticker under the hood tells the spec of the refrigerant (R-134a) and the correct amount to use (0.75 kg or 26.5 ounces). Too much is bad, as is too little.