[Steveski]

What is the most likely cause of the air not blowing fully out of the dash vents on high on a '79 T/A? A bear to get to the cause or air door in question?

[getmygoat]

Have you verified the vacuum hose from the engine to the firewall is connected well and there are no leaks in that hose?

You could also have a clogged check valve.... have you checked it?

[T.Weber]

Steveski,
Dumb question here but did you have the motor out of the car recently and if so did this problem occur after that? Also you said the A/C wasn't coming out of the vents are you getting cold out of floor heater vents? The reason I ask I had a 72 Cutlass that the A/C was coming out of the floor (heater vents) this problem started after I rebuilt my motor. Turns out there was a small plastic vacuum storage canister mounted on the firewall and I hooked the vacuum lines to the vacuum canister wrong. I flipped the hoses around and problem solved. Cold air out of the A/C vents.

[Geeto-Tiger]

Originally Posted by Steveski

What is the most likely cause of the air not blowing fully out of the dash vents on high on a '79 T/A? A bear to get to the cause or air door in question?

As the others have recommended, probably a vacuum leak. For regulatory reasons, it is mandatory that every vehicle provide a defrost function. As a result, most systems were designed to default to defrost when a vacuum leak occurred.

[engineer]

just feel where the air flow is coming from. Defrost, floor, dash or a combination. Sounds like vacuum, or doors or vacuum diaphram