Donald,
	Been there,done that.I have had both leaking heater core & valve.
The core I repaired by first finding the leak with compressed air and
holding it under water. Then I used flowable RTV used to fix
windshields. 
	I dismantled the valve , cleaned it well and sealed with blue
RTV. It still works too! While you are  at it, replace that threaded
nylon bleeder screw with an equivalent  steel bolt . It is not long for
this world.
	During the repairs I had the two hoses connected together using a
piece of wooden broom stick and two hose clamps. This prevents hot
coolant from by-passing the radiator.
Good luck.
Ken Lewis 
 kdlewis@juno.com
Kernersville NC
86 Doublecab Pickup Transporter
85 GL