How do I purge the air from a hot water furnace with zones
Well... the way that my father does it is that he isolates the zone from the other zones by shutting off the manual cutoffs.  Then attaches a pump to the drain cock on both sides of the zone, and runs the pump -- with luck you'll get the air out of the top of the line and back to the pump, and the auto-fill should replace the water.

Alternatively, you may have air separators at the top of each run, and you can open up the valve at the top to bleed those.