I like to use regular kitchen black pepper. It works well and won"t hardened up somewhere. It just seems to find the leak and stop it up. It has worked in a few of my tractors and the wifes car that had a coolant leak and I didn"t want to change the intake manifold gaskets.
I would start with a couple tablespoons and increase it if that doesn"t help. It doesn"t take much. It works fast. And, you don"t have to drain the coolant and run water with the product to carry it around.