Howdy! I have a 79 300SD with an A/C problem. Only the defrost button works on my A/C control panel. If my memory serves me correctly, this is due to a low, or non-existent, charge of freon. I am going to get a hold of someR12 and was wondering what is the proper way in which to chargethe system and how much refrigerant is needed? I look forward to any relpies and I thank you in advance!