I just bought a '76 280z with the stock L28E engine. After a few minutes of driving (either city or highway), the temp goes up and past the redline as the engine overheats. When I stop there is vapor escaping from one of the hoses (heater core I think). I tested my thermostat and it opens/closes fine. I flushed the radiator and unplugged its top hose. When I run the engine, no coolant comes out from that hose. Could it be my water pump, or something else?