[DonSor]

I thought that the cooling system in most cars are self air bleeding, that is, any air that is trapped in the system gets purged through the radiator cap and out the overflow. If the overflow is empty however, it will suck in air when the engine cools down. Instead of trying a little here and a little there I would do a complete check up of the cooling system. Make sure the thermostat works OK, that the radiator fan responds to the temperature sensor, that the heater valve including the vent louvers works in accordance with the switch settings.