the core may be air locked, and needs a flush to full it up with water, then the heat should work. maybe a heater hose has a cut in it and leaks only when it gets hot, when you take the hoses off the alt bracket, to flush the core, take a good look at the ends of the hoses for cuts or cracks, or plain old soggy ends that are loose fitting.
if all the hoses are ok, id pull the plugs out, and do a compression test. if you dont see coolant on the ground and see white smoke come out the tail pipes the engine is eating it.
