monvalue doesn't stop cold air
 

replaced the monovalue on 89 420sel, but still have cold air coming out of defrost at the same time warm air our of heater vent. what should i try next or shall i learn to live with it?

Next step is to test every vacuum actuator and all of the vacuum system that is underneath the dash. Sounds like a vacuum problem now. Make sure you test the vacuum solenoid valves too. Tney are located behind the pushbutton unit on the 300D - not too sure on your model. When I was going through my 300D I found two of these in need of replacement.

guy

You might also check the pushbutton unit. I've replaced it in two of my 126 cars, along with monovalves. The last pushbutton unit checked out OK but it was the problem. I was not getting heat into the footboxes. Also, check the aux. water pump for function as it has an effect on the pushbutton unit.

Happy hunting.