It would be almost impossible to make silver solder stick to cast iron. Brass works OK, as long as you get the metal CLEAN, which sometimes includes grinding the area around the crack smooth enough to get rid of the casting sand that's incorporated into the rough surface of the block. Preheating the area around the crack for a couple of inches on both sides of it will also keep the heat loss from preventing a good bond. A correct brazed repair should be smooth, with no areas or brass piled up on top without flowing smoothly toward the edges of the repair. it takea a LOT of talent to apply brass into a smooth, leakproof bead. I've only been doing it for 45 years, and I still screw up occasionally!
Jerry
