I have all the symptoms of a failing monovalve on my 94 E420 i.e. occassional and random full heat from the side vents when on full a/c. At the moment it is very intermittent but I know it will get much worse. My car is a late model with the combined monovalve/bypass pump which does not have a rebuild kit and which costs $300+ from Mercedes. Do I have any option but to replace this?. I have considered a manual on/off valve in the hot water inlet line to the monovalve for use in the summer.....is this advisable??