If I understand it correctly, the coolant moves to the overflow tank when the pressure/temps get higher. As the radiator/engine cools, a vacuum sucks the coolant back into the radiator from the overflow tank.

Our 2009 Corolla has high miles - almost 469,000 in about 38 months. Replaced the thermostat & radiator cap. Should we just suck it up & replace the radiator? Does not show any leakage. I am thinking maybe a hose is causing a vacuum leak, but it could come from the radiator as well, no?

Thank you for any ideas.