Glad you called me out on that diesel vacuum pump mistake, I forgot it was a diesel, OOPS!
I don't have a TCC book in front of me at the moment, so I'll have to look at one tomorrow to be more specific.
Start simple, Check fuses - even if it looks OK, replace it fuses are cheap!
You could still have a vacuum leak in the system, or an electrical problem in the pushbutton assembly. You can order a manual for the TCC system from your MB dealer or Baum Tools 1-800-848-6657. The manual will have electrical diagrams and vacuum diagrams as well as function tests and theory of operation.
Based on what you said regarding the heater valve, it sounds like it leaks internaly, so replace it.
I will look into it some more tomorrow and let you know if I think of anything else that might help.