Like Geko says, I would flush it with tap water, then try to get rid of as much of the tap water as possible with compressed air, then flush the last out with distilled water.

 

Actually the best way to flush is pull the drain plugs in the side of the block, pull the lower rad hose, and disconnect both heater hoses. Shove a funnel in one heater hose and put the other in a position that will drain water out. Pour in a liter of distilled water and voila`, flushed.