91 Previa heater

I just bought a 91 Previa with 180K mikes. It takes a longer than normal for the engine to heat up to normal temperature when I start in the morning, but once the gauge reaches about mid range it stays there. The heater takes even longer to start work. Once it starts to blow hot air if I turn on the fan to full speed the air starts get cold.

I replaced the thermostat, checked the heater valve, and checked the heater core for blockage. It seems to me that the water is not circulating through the heater core. I started the engine and disconnected the water return line form the heater and there is no flow.

Any body has any suggestions or ideas. Thanks