[Trevor78]

It has a rubber grommet that slides in from the side, often dried out and hard.

76 Camaro AC heater box6 by Ben, on Flickr

I'm not sure what you've tried, and can't recall how mine was which is modified anyway, but there's adjustments in the length of the control rods, location of the pivot brackets, and the manual cable length from the dash. From memory one is closed under vacuum but fights the spring tension so will open again just sitting there.

[GoreMaker]

Originally Posted by Trevor78

I'm not sure what you've tried, and can't recall how mine was which is modified anyway, but there's adjustments in the length of the control rods, location of the pivot brackets, and the manual cable length from the dash. From memory one is closed under vacuum but fights the spring tension so will open again just sitting there.

This specific door is held in place in the middle of its travel by a spring. It's sprung in both directions. Vacuum on the yellow tube (Off, Heat, Defrost) directs all air through the heater core. Vacuum on the white tube (Max, Norm, Vent) bypasses the heater core. No vacuum on either tube (Bi-Level) causes air to flow through both paths because the spring keeps it there. The vacuum canister doesn't have enough total range to seal the door completely in both directions no matter how I adjust it, so I'm wondering if it should be sealed when bypassing the heater core, or when flowing through it. I'm leaning towards sealing off the heater core so that air from the evaporator doesn't get heated unnecessarily when the AC is on. It shouldn't really matter if a small amount of unheated air gets blended with the heated air when the heater's on.

[Trevor78]

There's a return spring already built into the arrangement of rigging where the levers are, where cable attaches. There's also a return spring that holds the defroster/floor door sealed when not under vacuum.