Every time I do something with the coolant I have to "burp" the cooling system. You'll find this information several places if you do a search.

Take the cap off and elevate the car so that the overflow tank or wherever you fill, is the highest spot in the system. Mine's on the right side, so I pull the front right wheel onto the curb.

Let the car warm up to where the thermostat opens. Mine is about 80 C.

When it is warm, I rev it to about 2500 for a few minutes. That lets the water pump spin pretty good and the air will want to leave the system at the highest spot.

This works for me. What I usually see when there is a bubble is a big fluctuation at the temp gauge that has little relation to driving conditions, that is, slow driving can show high or low temps and the gauge swings much quicker than when there are no hot spots.

YMMV