So after putting it off for more years than I will admit here, I finally replaced the bad heater core in my 1970 Vista Cruiser with factory A/C. (I live in Florida so it was easy to do without.)
That went okay and I now have heat in the cabin properly regulated by the bottom slider switch.
But...
Now when I move the top slider switch between Vent, Heat, Max, Normal, and Bilevel, air only comes out of the heat vent in the bottom. Defrost still works.
There were only two vacuum hoses to disconnect to get the heater box out of the car, and both are reconnected (in the right order!) so I'm at a loss as to why it does not work now. The chassis service manual has no troubleshooting tips for this problem.
Any ideas on what I screwed up here?

Thanks.

 

The times I had heat from the floor only it was due to loss of vacuum from the intake manifold to the vacuum ball on the firewall. Check to ensure both vacuum hoses to the ball are intact and connected.

 

Yes, they are still hooked up. I'll run a vacuum test on them, tomorrow, though.

 

Could be loss of vacuum at the dash switch. I’m not familiar with the behind dash vacuum connections.

 

If it worked before and now doesn't, the most likely cause is a vacuum hose that is either not connected or incorrectly connected. If vacuum to the system is lost, it defaults to air out the floor and defroster vents.