You don't fill it to the brim, hanggang sa neck lang nung radiator filler neck. The brim is where the cap fastens itself. The valve for the radiator cap reaches only the neck, not the brim, hence the overflow mechanism is activated and pressure is relieved via the radiator cap. HTH

Also for those with expansion tanks/reservoirs, this works the same. Do not overfill, only to the "MAX" line. This allows excess coolant from the radiator (remember, it expands when it gets hot) to transfer to the reservoir in case it gets too hot, and when it cools down coolant goes back. That's why you have to leave some space and not overfill the reservoir.