keel cooling air lock problem

[Spice of Life]

I have a 1974 Hundested single cylinder 27 hp hot bulb engine in my 50 ft steel lug rigged schooner. For those not familiar check Utube it burns diesel. My problem is I think I am getting air locked into my keel cooler, so the water over flows from the reservoir, hot. I flushed the system with compressed air and refilled with fresh water only, no glycol, retightened all clamps and I put a hot water heating system auto air vent at almost the highest point in the hose. The vented reservoir is 2 ft below the auto air valve. Started engine, watched see thru flow paddle, air bubbles for a few min, topped up water after 45 minutes running the reservoir over flowed again hot.
Anybody got experience with such problems?