Or, power wash it from the drain up to the t-stat, then the other way. No acid's unless its made for block flushing. I have flushed radiators with regular coolant system cleaners using a water pump and hoses. I just let it set on the floor, ran hoses to the pump, and let it set and run for a day. Being cold water it took longer, but with no engine to use, this worked really good. You could do the same, plumb a hose from the drain to a pump, and then dump it back into the top. A day of running would really help.
Joe
