You have replaced the heater (mono) valve, but have you checked the signal to it? I don't know, but I expect that it gets a signal from the pushbutton control unit as does the 124 cars. On the 124 cars a voltage to the valve SHUTS IT OFF. Use a voltmeter at the valve to see if it is getting voltage.

Surely someone here knows if this works in a normally open position or not.

Good luck,