Looking into this.. how about having two shut off valves, one for each hose thus shutting off ALL pressure to the heater core in the summer? Any takers?
Well, it's a zero pressure system so I don't think it's going to make any difference at all in the health and longevity of your heater core.
My '39 Packard has a factory installed heater and factory installed shutoff valve on the inlet hose. If the designers thought a second shutoff valve would provide any benefit, I'm sure they would have installed one.
