Juan, remove the cap from the expansion tank (with the engine cold) and run the engine for 10 - 15 minutes ... that will allow air trapped in the cooling system to escape.

The upper hose connections on the Behr radiators become brittle with age, develop circumferential cracks and break off. You may want to remove the upper radiator hose and closely inspect to see if the hose connection is cracked. Some folks do a crude fix with a piece of copper inserted inside the hose connection ... better to replace the radiator and preclude having the connection break off at the most inopportune time and leaving you stranded.

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Fred Hoelzle