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The heater must have some kind of valve to control heat. Either by controlling the coolant to the heater or bypassing the hot air from the heater core (not likely).
The vacuum lines may supply vacuum to operate the heater control valve. Not sure about the Schrader valve. Some cars run a temp-sensing bulb into the climate control vent. This bulb helps control heater temp by opening and closing the coolant valve.
Suggest you follow the coolant lines to the heater core and see what you find. There should be a valve somewhere to control the coolant flow. Guessing you will need to go under the dash to find this valve.
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