The 124 chassis cars such as yours do not have the problematic monovalve. The valve itself is different.
There is a little grill next to the rear view mirror. With the ignition on, see if smoke will draw into that grill. There is a little fan that draws air temperature over the sensor. When it quits, the a/c will do all sorts of strange things.
If it does draw smoke, after running feel of both heater hoses. If they are both the same temperature after running the car from cold for five minutes, then the pushbutton control unit may be opening the heater valve. This would be a possibility if the a/c is putting out HOT air, not just less than cold air.