From what I have been able to gather the system is designed to allow 20% fresh air even when in recirculate mode. I have read about a modification for a 123 chassis, but that does not seem to be applicable to the 560SL. I have also read that when you rotate the temp. wheel to min (to where it clicks) you would get full recirculate. I do wonder however if that means 100% recirulate or rather the 80%/20% mix. I also do not see much of a change between having the recirculate button on or off. There are two hoses going to the back of that switch, but there are 3 holes and I am not sure if I have the hoses in the right holes. My shop manual a/c vacuum schematic is not clear on which holes to use. As far as the vent temp, I do not have a thermometer to measure it, but it is not really really cold like my other two cars. Only maybe around 65 degrees as a wild guess. I have replaced the mono valve last winter as I was not getting heat. That did not look bad, but I did get heat afterward.