[Bruce]

I've posted this before but, as it came from a vetern radiator rebuilder, I'll restate. Use two cups or so of liquid dishwasher detergent and clear water. Run it for a week or a hundred miles -- it won't hurt. Then drain it and pour it down a fence post -- no harmful stuff to make AlGore get his panties in a bunch. I did it a couple times and the "clear" water came out black the first time ... gray the second.
Oh, and the radiator guy said to backflush, as the oily stuff from greasing the water pump floats on water and deposits at the top -- not the bottom -- of the tubes and in the filler neck.