I'm helping a friend with his '85 euro 280 SL. The A/C didn't work when he got the car, and a quick check revealed that it was completely empty of freon. Bypassing the low pressure switch causes the compressor (York piston type) to run when the temp wheels on the console are set to the blue zone and the a/c rocker switch is on, and the compressor is smooth and quiet. My questions are: are these systems efficient when they work? (interior seems like a small area to cool) Are there any known weak spots where leaks frequently occur? What would be involved in converting this system to R-134, and would it function adequately if so converted? Thanks in advance for any advice!