bill1949,
It appears to me that hose "B" would be considered the supply hose. It comes out of a nipple on the water pump, through the heater core, then on to the thermostatic valve and completes the trip ending up in the thermostat housing, on the engine side of the thermostat.
The water pump is always trying to push the water through the heater core. It can only be stopped by the valve, or a thermostat that hasn't opened yet.
The heater core probably wouldn't 'air lock' as it has the positive pressure of the water pump behind it.
The heater valve must be turned ON to allow the water through. It's a open system except for that valve and the thermostat...you could always just bypass the valve and see what develops.
If you print the picture, it becomes much more readable and clear. Double check the hoses and give it another try...it's a pretty straightforward deal.
Stuart
