I flushed the cooling system on my old Africa Twin this week, when i drained the expansion tank it stayed blue up to the high level line so wouldn't have known if it was low just by looking at it. I started thinking of ways i could clean it, went through the wife's cleaning cupboard and after trying a few different detergents unsuccessfully i found this stuff
Its quite a coarse power so got an idea of how it might work, poured some into the expansion tank added some hot water and spent the next few minutes shaking it vigorously and to my surprise the stain started to disappear flushed it out with hot water and repeated the process. I think i got a good result with it, i just wish i had a before photo to show how bad it was.