Did you ever try bleeding the entire new loop at the boiler with the makeup water pressure regulator in the "full open" position? You can't put it on full pressure with the system closed, but if your drain valve is open, you can. The highest pressure you mentioned is 20 PSI, so I guess you did not put in in full pressure mode. The water goes through the system MUCH faster when unregulated than when it is in normal mode. that's a lot of running up and down stairs by yourself. The circulator does not have to run to do this as far as I know, but the zone valve has to be open if there is one for the new loop.