Leaking Heater core in my 1955 Chevy

Check your heater hose and or clamps. It may have a small crack that self seals itself when the hose gets hot and expands. It also could be dripping and evaporating until the engine is turned off and the heater parts ae cooled down so it does not evaporate. You also may have a leak in your heater core itself. These old ones were made of copper and put together with silver solder and over the years could develop a crack or leak. These should be easy to remove and have checked. The hoses are an easy start to replace. If new hoses and clamps do not fix it, next is pulling the heater core. It may be cheaper to buy a new one, but these old ones are also an easy fix if you or someone know how to do it. Find someone in a radiator shop that has gray hair and winkles, He should know.