I have also found that after a flush and drain that it is well worth the investment to refill the system with distilled water.
I get it locally at K-Mart for 59 cents a gallon.
It seems to keep the scale buildup away.
Just be sure to drain or flush the block as well.
Just my .02.
PS I work for a Benz dealer and we have a citric acid flush that dissolves all scale and sediment. It is part # 000-989-10-25.
1965 Mustang coupe
1983 Monte Carlo Conv.
Yeah, thats right...Convertible