Heater Core Replaced, Now Blows Cold Air

Problem solved. There was a 'restrictor' in each hose connection for the new core, which was blocking coolant flow. Don’t know exactly what this restrictor was intended to do (or why it wasn’t obvious the lines were blocked with them in); but with them removed the heater system is now working. Not entirely thrilled with the mechanic, as I can’t imagine why this wasn’t noticed with the core installation. But no damage done, all is OK now.

Thanks for the suggestions (I learned a little about the vacuum pump),