I don't have a whole lot of experience with the older cars, but my understanding is that when the monovalve fails, it fails open for safety reasons allowing coolant to flow for defrost function. Feel the heater hoses when the engine is at operating temp to see if coolant is moving back there (don't burn yourself of course). You might have a clogged heater core. Nothing else is really coming to my head, but when I think of something I will post it. Good luck.