Well it has been over a year since Part 1 was posted.
With lots of free time from the shelter in place orders, the previously bypassed heater core is now operational.
Finally there is heat available in the cabin, no more shivering in the car during cold weather.

The metal pipe lines for coolant flow from the engine to the servo (lower pipe on MB part number 116 830 34 150),
servo to heater core (upper pipe on MB part number 116 830 34 15), and servo to water pump (MB part number 116 830 35 15) were severely corroded and needed to be replaced.

Unfortunately, both parts are NLA, but I figured out a workaround.
Months ago, I remediated two used pipes pulled from a car at Pick-n-Pull.
They were soaked in EvapoRust for about a month.
The interiors of the pipes were scoured with a long brass bristled bottle brush.
The exterior ends were scrubbed with a steel wire brush.
Then they were powder-coated with a shiny black finish.

Removed the leaking Servo Valve (aka Evil Servo) (000 830 03 84), removed the seized Auxiliary Pump (000 835 69 64),
removed the rusty Pipe Line from servo to water pump (116 830 35 15), and removed the rusty Pipe Line between engine and Servo (116 830 34 15).

Attached Thumbnails

         

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022