Some of it should come off, bit by bit (ever so slowly), everytime that you flush the system.
What should be going on (someone correct me if I'm off) is that this residue (copper oxide) will get dissolved into the water, until it's saturated. Not hard to do when you start with distilled water.
You could try flushing out the rad with a warm vinegar solution to accelerate the process; just set it up in a loop with a pump and airtrap only, and let it run overnight. Not sure how you'd keep it warm that long though.
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