Join Date: Aug 2013
Location: Northern California.
I recently replaced the leaking heater control valve in my GTV. The original heater core (original owner of car) tested good, but you may wish to replace yours anyway. Checking a new heater core (if purchased) for leaks before installation, is a good idea.
As already mentioned, it's a job that takes time to accomplish. I first removed the dash and console, then removed the heater box assembly from the car for inspection and control valve replacement.
While everything is apart, you may wish to consider replacing the heater hoses and clamps, as well.