I don't think your car has the monovalve. I believe that it has a simple heater valve like found in a 124 car. If it does indeed work like a 124 car, and you did not get coolant flow when disconnecting the valve the problem is elsewhere, most likely the pushbutton control unit.
If it does indeed have the monovalve type sytem, there will be four screws on a square plate at the top of the valve. If it is like the 124, it will be made in such a way that it cannot be disassembled.
I was hopeful that someone who was familiar with your big body car would chime in and let us know if your system is like the 124 car, or like the monovalved 123 car.