Heater works while accelerating, blows cold when release gas

All the McParts stores sell a Prestone flush kit that includes various sized couplings to allow attaching a garden hose to a heater hose. Those adapters make great bleeders for the cooling system. Installed in the highest point in a heater hose, top off the radiator, start the engine (cold), loosen the cap on the adapter and watch for coolant to dribble out. The air will be gone.