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NW Ohio,near Findlay | May be vacuum issue, don't know. That old of a system, first thing I would do is backflush the heater core both ways. Check the hoses once warm, see it one is colder than the other. Flow test is your best test. Unhook both hoses, connect to garden hose, see if restricted.
If you think your blend doors are working and not enough water flow, just install a inline quarter turn valve, and do away with any other control valves. Good auto, truck places have them, you can control them by installing good old cable into the cab. On my Louisville truck, I just manually shut it off under hood in summer, and keep it open in fall and winter, too hot, don't run fan, work simple.
Don't want to be a Debbie Downer, but it may be more serious problem it no heat going down the road, head gasket blown. First thing you will notice on a blown head gasket is no heat, water doesn't circulate with compression air in water. You can check this by removing radiator cap slowly, see if bubbles in water. If you run truck with cap loose to let compression out and heater works, blown gasket or cracked heads. Boy do I know when wife comes home complaining heater doesn't work on my Olds Diesels, it's a bad day, head gaskets again, and again next year, seems like they always blew going into the winter.
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‘77 Ford f150 heater issue
