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Very common of having the radiator filled too full. As long as the tubes are covered with water you will be fine. If you are adding water back up to full each time out you may never stop having water flow out of the radiator. Keep an eye on it and let it find the level that it likes. Use a meat thermometer placed in the water to make sure that it is not actually over heating. As Mitch said you should not have water coming out of the cap regardless. Any overflow should be out the tube at the bottom of the radiator. Your cap is not sealing.