I had my factory ac, 66/427 coupe, recharged today. The system seems to work, compressor kicks in when air lever is pulled out. Air temp at idle or slow speed is 50 degrees, at highway speeds it lowers to 40 degrees. It never really cools the car. It was previously changed to R134a, should I change it back to R12 or is this the best it gets? I put a small thermoter in the center dash duct to measure temps. Please offer any suggestions.