I've never had air in the cooling system. After draining the old coolant, I pour a 50/50 mixture into the radiator slowly until I see it above the radiator core, then with the cap off, I start the engine and monitor the temperature gauge and watch for movement in the coolant. When the thermostat opens, the coolant level will drop in the radiator upper tank, then I add more 50/50 mixture in very slowly until it's about 1/4 inch from the top of the tank. When it stabilizes at that level, then install the radiator cap. Never had air in the system.