Let me try and describe where the leak is. The heater core under the
rear seat has 2 hoses to it. These attach to a small block that has a
valve and lever on it. The valve bolts onto the housing for the rear
heater. It's leaking where the join to the plastic housing is. I don't
see a gasket in there but I've not dismantled it because the leak is
slow enough to allow me to drive the van. 

Can I get a gasket? Should it have one? Could I use one of the liquid
gaskets? If so, what do you recommend, it obviously gets hot from the
coolant flowing into the heater core.

Thank you
Melvin