Why no Vacuum at Heater Valve?

1970 300 Hurst with 440 auto. Saved from junkyard. Dash and firewall was rotted, but I got almost everything to work. When I bought it over a decade ago, heater/ac buttons were stuck, so I kept pushing/lubing them until they operated smoothly again. In the last few months, I haven't heard the "pssshhhhh" sound anymore when I go from "off" to push in the "heat" button, although fan blows hot air. My A/C has never worked but I noticed it blows hot air too. Around the same time, I...

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