take it out and take it all apart. The valve and heater core are both in there along with a 90 degree elbow (rubber heater hose). Any one of them can and will leak! I took my core/valve assembly out of the box and then hooked it up to the engine outside the vehicle (heater hoses are long enough to do this) and started the engine up to see what was leaking! Your valve could be stuck closed or your core could be plugged up as well. Either way, you have to take it all out to test and then fix.
