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2. Once empty, I poured in the entire contents of the Prestone Flush and topped it off with water. Then, I fired up the truck, let it come up to temp, turned on the heater, and let it run for 20 minutes. After 20 minutes, I opened the drain plug once again. Caution, it will be HOT, and let it drain. I refilled the radiator with water and repeated the cycle.