This type of leak can be fixed with a quality sealant added to the cooling system without it first being blown out by the gases blowing back from the cylinders.

A couple of solutions are:

1. Remove a top hose from the radiator and pour the sealant into the hose so that it enters the cooling system directly.

2. Drain a few quarts of coolant/water from the system, mix this with the sealant and then pour it back into the system. This helps deliver the sealant right inside the cooling system, giving it the best chance of making a repair.

Even though there is pressure coming into the cooling system, there is almost always a small movement of coolant back across the crack as the pistons move up and down within the cylinder. The sealant is drawn into the crack and is then cured as the hot gases pass back through the crack. 

This solution is brought to you by K-Seal, www.solv-tec.com.