Thanks for the info. I did disconnect both Radiator coolant lines & test for flow. I am getting flow.
The test port described in original listing is a 12mm Bolt just fwd of the Vacuum Modulator.
This procedure has been shown in previous posts such as
http://www.peachparts.com/shopforum/diesel-discussion/190197-w123-modulator-adjustment-diy-measure-right-way.html
Hope this helps someone suggest new thoughts on source of problem