Heater No Longer Works

I have a 1978 Ford Granada with a 250 inline 6 engine with factory a/c. Last Thursday I turned on the heater but there was no heat. The water valve is plastic with a hose nipple. I connected a vacuum gauge and
found that there is no vacuum to actuate the valve. Today I connected a vacuum to the valve but I still don't have heat. I just want to know if the problem can be the heater core or the heater temperature adjust before I take out the water valve.
By moving the lever to "heat" how is the vacuum connected to the valve? Is it mechanical or electrical?
The a/c cools but if I turn the lever to "hot" there is no heat.
Any help would be appreciated.
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azazel scratch wrote:

First, verify that the thermostat is not stuck open. If there is coolant flow in the radiator with the engine cold, change the thermostat. All of the controls are vacuum operated. Does the airflow change with the control ie;defrost, vent, floor, etc? The water valve should be open with no vacuum applied. It is closed when the mode selector is in the Max AC position and possibly the AC position. With the engine running and the mode lever in the Floor position,feel the hoses on both sides of the valve, they should both be hot. If both are hot, feel the hoses on the heater core, again both should be hot. IIRC the temp control is still cable operated on your car. Follow the cable to the blend door on the heater box and check to see if it moves with the lever.
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