[engineer]

some one needs to check something. Most of the older 65-67 A body cars do not have water valves in the heater hose lines. The ones on like 71-72 GPs only work if the AC is in the max. position. The idea of an air pocket in the system does not apply since that would mean the water pump with the bypass would be vapor locked, or cavitating which is not possible if you look at where the hot water return is compared to the suction on the timing chain cover. You are adding a lot of heat back into the pump. GM always had water flow through the heater. In fact, if an enigne is experiencing overheating, it is recommended to put the heat on high to help remove heat.