In-dash air conditioner - Worth converting or fixing?

So there is a new wrinkle. I moved around the wires and jumped the clutch on the compressor. It worked fine. Then I tested inside the cabin and when I switch on the A/C, the compressor engages. So the electrical looks like its working. I thought the system was totally empty, but when I started unscrewing the receiver/dryer (so I could replace), I heard a tell tale hiss of venting gas. Could that just be compressed air from when the compressor was running or is that venting sure to be some remaining R12?

Jose