Yup. I haven't looked at it in 100 years, but basically, when you go from cold toward hot (as soon as you come off COLD) with the temperature lever, it applies vacuum or releases vacuum to the valve depending on the setup to open the valve to allow water to flow through the heater core. When you are in MAX or all the way COLD position with the A/C, the valve applies/releases vacuum to close the valve to stop flow through the heater core.
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