Flush it aggressively to get any sediment out, and I don't mean fill, heat cycle, dump; I mean put the hose to it with the drain pulled and run some FLOW though it. Next, optionally, run a treatment option of your choice, dilute vinegar and time, commercial radiator flush, CLR if you wanna go REALLY aggressive, etc. Once done, put distilled water in it, use a track safe/legal additive package like Cool-Aide/etc as a corrosion inhibitor, and change the water more frequently going forward.