how do i bleed air out of the cooling system on a 2005 dodge caravan 3.8L

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i replace a new radiator and i believe air is in the cooling system i can fill the radiator up in a few minutes the water go down in the radiator what cause the water to keep on gown down in the radiator? i drove the vehicle and i had my scan tool hook up and i was checking live data the temperture gauge went up to the red mark and my scanner show 251 but when i cut the engine off i didn't hear the water boiling in the radiator what cause the gauge to go up to the red mark? seem like the faster i drive the higher the gauge go up but when i slow down the gauge come back down to 1/2 mark if the enging was overheating wouldn't the water be boiling in the radiator what cause the gauge to go into the red?