W126 has no air blend door. The monovalve and A/C compressor are cycled to control vent temps.

If the Monovalve is working properly, it effectively closes off ALL coolant flow through the heater core when in cooling mode.

Adding a ball valve is a pretty half-assed (and unnecessary) solution. Repair/replace the monovalve control circuit or the monovalve cartridge. The monovalve coil is energized to close it, if the pushbutton unit is acting up it may not be energizing the monovalve.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]