I was through the same problem with my black 1994 S420 (at 135,000 miles). I bit the bullet and bought a new duo-valve for over $400.00. It worked great...for about a month! Because I about had it with the hot air out the side vents, I unfastened the main hose going to the duo-valve and put a 3/8ths to 1/2 socket adapter in the hose about 3 inches from the end and put a hose clamp around it. After that, I had plenty of cold air from all vents. Once winter was on its way, I simply removed the socket adapter and enjoyed a warm and cozy car during the cold. The procedure is not very unsophisticated....but it is a sure thing. I sold the car a couple of years ago and the guy I sold it to does the same thing as the seasons change. Chasing a duo-valve is expensive...but fix was cheap.