Bleed air out cooling system

The problem you may be having is one where the radiator filler cap is actually lower than the heater core in your car, meaning that there will be an air lock near the heater core. Sometimes one might park the front wheels on a elevated curb or car ramps to raise the fill point of the radiator, use wheel chocks to prevent car from unwanted movement. Then start the car and allow it to warm up to operating temperature adding coolant as needed until radiator is completely full. This doesn't always work in every situation and may require someone familiar on how to do this. This is very important to have this done properly or your car will overheat and that can cause engine failure. And I can never stress enough to wear safety glasses and gloves, use jack stands and always use caution while attempting repairs. It's fun and rewarding to fix your car but not to get hurt.

The air within the radiator should bleed out naturally after you've run the car up to operating temperature. Allow the car to cool off before removing the radiator cap and refill until full; as the radiator fluid circulates throughout the system, the excess air will rise to its highest point which is the expansion tank or radiator cap. Most radiator caps are located on the expansion tank but do not open the cap when hot due to the pressure build up within the system; if you're not sure if it's cool enough use a large rag or towel and cover the cap in case of pressure release.