I have had good luck using CLR. Flush with water, Avoid high pressure air you can easily blow out a weak core. Pour the CLR in through the heater hoses, let it sit, flush with water again. I would flush it into a white bucket and stop once the sediment stops coming out.

Good Luck

Scott 1956 Dodge Custom Royal Lancer (408 Stroker, 4 Wheel Disc Brakes, Rack & Pinion, 6 speed) 2002 Dodge Ram 1500 Quad Cab 1976 Corvette