It seems that my factory vacuum operated shut off valve is eithe. Broken inside or just doesn't seal anymore. The vacuum is working fine but I still get a reduction in a/c output. (Surface of evaporator is 34/35 degrees but panel temps never fall below 55-60)

What I would like to know is if I can install a manual shut off to block 100% of flow to the heater core? I don't know if the factory vacuum operated valve was ever designed to block flow 100% or if it was meant to flow a little bit for thermostat bypass reasons. Any input would be appreciate.

 

No problem at all, I have a ball valve in my heater hose.
I shut off the valve during AC season and open it back up when I need the heater in winter.

 

Every semi I've ever ran had some form of shutoff valve either ball or gate; my brothers 12v block that came out of a bus had a gate valve.

 

I installed manual shut off valve on my motor as well. Check out post #707 in my build thread....

https://www.dieseltruckresource.com/...8/#post3250003