What did I do to my new track bike last night? Flushed the coolant system. It took 16 litres and starting/warming/stopping it like 30 times to get it flushed to the point where I was satisfied with it for now. Still not 100% clean, but I'll run it to clean the last of the grunge. Can anyone tell my how the hell an aluminum cooling system can build up brown, nasty, chunky rust? I bought this bike from my friend and when we got it we flushed the system because it was full of antifreeze. We put in distilled water with water wetter. Now it looked like, well, nastiness. I've never seen any bike do this before. I've run water/wetter for a decade in all my bikes without issues. Even if there was still an air pocket floating around in the system for a few years how could it rust brown with aluminum?