[Tom Wesenberg]

I would fill the radiator to just over the tops of the tubes, as Mitch said, then drive it and see if any more coolant goes out the overflow. If it does, then I'd remove the radiator, lay it flat with the front down and your hand over the cap opening, then fill it with water. Now quickly turn it upside down and remove you hand and see if any rust flakes come out. On most radiators it doesn't take many rust flakes to block off enough tubes to cause the coolant to fill the top tank and go out the overflow. I've been through this exact thing a year ago, and it all started when the built up rust in the rear of the block decided to let loose and block the radiator tubes.