You need only one valve in either heater core line.

 

Just picked up a brass gate valve at the auto parts store, had barbed fittings on either end and hose clamped it in. Been in for 3 years, no problems.

 

Put the valve on the line from the water pump to the core. Otherwise you will put unnecessary pressure in the core.

 

Thank you all for your help and input.

Here is the update. Basically removed both in/out heater core hoses from the firewall on the engine side. Capped both brass extensions on the firewall where hoses had been. Connected both core hoses together to maintain closed loop on
engine side of firewall. This was done to maintain minimal pressure on the
hoses and completely remove heat from interior cab.

Test drove and interior AC is noticeably better. But not totally acceptable,
especially at speed.

As the AC/Heating control assembly all seem to work though temperature gradient lever now has no heat (until fall revertment), I am considering wrapping/insulating the cold aluminum line in the engine department as a test.

Any thoughts?

Thanks again,
94DakLover