how do you know the monovalve is working? its the weakest link in the system, i'd thoroughly rule it out before doing complicated flushing of this and that. they break easily. if yours is still original to the car it could very well be shot. they can fail in multiple ways such that you get full heat all the time, or no heat, or weak heat, etc. even if you're getting voltage to it the valve itself could be faulty

__________________
1981 NA 300D 310k miles