Shawn Miller
The heater cores seldom go bad, as the tubes in the core are quite
thick. Take it out, put it in a pan of water and check it with air
pressure.
The heater control valve rubber control diaphragm is a constant source
of coolant leaks, because the rubber cracks.
It is a bit of a project, but NAPA auto parts sold a repair kit, for
about $10. Ask for Balkamp 660-1000.
Good luck.