NC hybrid setup/coolant blows heater core/how to avoid?

[Howard Wilkinson]

It would seem to me that no matter what you do as far as restriction,
as soon as the key is shut off, the increase in cooling system
pressure will be equal throughout the system.  I find it hard to
imagine that the heater core would be subjected to enough increased
pressure from the water pump to be an issue.  The water pump mainly
creates a pressure differential throughout the system so that coolant
can circulate..... I suspect that there is very little if any increase
relative to atmospheric pressure over what the static system pressure
is.  A good heater core should be capable of withstanding any internal
cooling system pressure likely to be seen in an automobile with a
properly functioning pressure cap.         H.W.