Rockauto has it. It is the first or the last in the link. It's the brown 3 port (ported vacuum) switch.
https://www.rockauto.com/en/catalog...ical-switch+&+relay,ported+vacuum+switch,4604
I'm in California and have to have bi-annual smog tests. My truck has been off the road for about 12 years prior to my purchase last year. I just replaced all emissions parts on my truck. The left circle is your broken port/missing vacuum line.
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You are also the vacuum line that hooks up to the EFE valve, green circle. It hooks up to the bottom of the 2 port valve near the thermostat. When I was fixing my truck, I found the vacuum pod, red circle, for the heat stove was bad. I replaced it with a part from The Parts Place (Vacuum motor CW8422Z / AC 6484245). I had to modify a little, but it works perfect.
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My last truck was a '79 K15 with a fully smogged 350. Major dog. I did a couple of things to helped it. I installed the tall air cleaner lid. The air filter is 2" taller. It helped when the secondaries opened. I also cut the end off the snorkel adapter in the radiator support, creating a Ram Air effect. It ran better with this set up than with an open element air cleaner.
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