I think there is some "mis-information" here... the valve only stops the flow of coolant. But because the other heater hose does not have a valve, the heater core is always under pressue. So if the core leaks or ruptures, this wil not stop the coolant from leaking out.
Just thought I should clear up that point.
If that was true the suck side of your vacuum cleaner would be a compressor instead of a vacuum. Same principle. Putting a valve on the push side won't stop a leaking core from dripping I agree BUT it will take the pressure off & possibly save a core that is weak but not leaking yet.
