My chevelle has a no water circulation into radiator .I've replaced the thermostat,the radiator is fairly new using anti-freeze,the engine gets hot enough to open the thermostat,but water is not ciculating.The cap is off while watching the water flow.The only difference is that the water level rises in the radiator and comes out the overflow neck.The water pump is not leaking.Can water pumps go bad without leaking.And if so,how can you test with the pump on the car? Thanks,Randy