AZ442

Can someone advise me how the vacuum actuator for the vent is designed to work. Mine will not move by vacuum and has vacuum at different settings of the HVAC control (2 vacuum lines to it)

When driving with the HVAC off I have hot air coming from that side with no hot water in the heater core.

It is in the down position all the time.

Bluevista

If the '70 is the same as '71 here's a schematic from my CSM that shows the vent position.
The text below it is explaining the AC max position.
My book show 4 vac lines, There are two to operate the outside air door, one that goes to the vac tank and one to the hot water valve.
The air door should be open in all control positions except AC max where it splits it 80-20 between outside and inside air, 20 percent outside.

No hot water in the core for sure?
I would guess something with that #4 door otherwise, maybe the outside air door isn't opening?

BlackGold

For what it's worth, in either '71 or '72, they switched the hot water control valve from normally open to normally closed -- or vice versa; I can never remember.

A Chassis Service Manual of the right year would be your friend.