"What about blowing through the petcock with compressed air to get the junk out of the way?"
You could do that but it still doesn't get the crud out of the block. It would likely just re-deposit itself there again or somewhere else anyway. The only way I know of to get rid of it for certain is to tear it down to the point where you can dip the block in cleaning solution (some call it boiling it)and then run long spiral brushes throught the jacket. There may be some commercial solvent you can run through the cooling system that will help loosen it but I am unaware of it. Perhaps someone else might know.
