Success!!!

So, two issues have been addressed: Does the pump need to be "primed", and is the pump positional (ie, will not function unless it is positioned below the reservoir).

As Shortsguy1 states, it needs to be primed. I fashioned a tubing connected to a bottle of water, squeezed, and purged all air from the system. Thing fired right up (washers came back to life with force).

Attached Thumbnails

 

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Current:
1971 Mercedes Benz 250 (Euro Spec)
1972 Mercedes Benz 250 (US Spec)
Past:
1972 Mercedes Benz 280