Search for and PROPERLY BLEED
You have to do it cold with the engine off but key turned to on position with heater turned up to max so the aux. pump circulates coolant through heater core. Have the front of the car jacked up and fill through the bleeder screw on the thermostat housing. Open bleed valve at the reservoir and fill until no bubbles come out of the reservoir bleeder. Close everything up. Get up to operating temp, let cool down and recheck level in reservoir after complete cool down. Do not open cap when still hot or you may introduce air back into the system.
That's not the only way, but you can search to see alternatives.
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