It sounds as if you are talking about the monovalve as found in the 123 and 126 cars. acha is working on a 124 car. On the 124 car the heater valve is not a monovalve even though many folks call it that. It is a plastic heater valve that is not rebuildable.
I'm sure that your information will be helpful for those with a true monovalve though. The $30 repair kit is a five minute job to replace.
Have a great day,