Radiator reserve tank

Pinch the return hose going to the radiator, remove any existing anti freeze. Using a light detergent clean out the tank. Sometime you can get most of the junk out with just flushing it but for heaverier spots you can use a old toothbrush or a brush with a long handle that fits in the tank easily. Once the tank is cleaned flush with clean water to remove any soap. Flush it out well. Then refill with anti freeze