Welcome to the club of frustrated 700/900 owners with impaired air vents.

There are doors (flaps) inside the center console air distribution box that control air flow to the defrost, face and floor vent ducts as well as the cabin air recirculation flap (cuts off 80% of fresh air intake from the cowling vents below the windshield).

They're operated by three spring loaded vacuum servos (some call them actuators or vacuum motors) connected by colored pneumatic tubes to the heater control console (dash control knob). The servos are round plastic disk chambers in a cluster on the left side of the air distribution box. The rubber diaphragm in the middle operates a lever rod that's connected to the spring loaded door vent. The rubber gets ripe with age and eventually starts to split and leak. Initially the air flaps won't stay open without strong engine vacuum (such as when vacuum drops off during acceleration or going up a hill). Eventually they won't operate at all.

When one leaks it affects vacuum pressure in the whole system, so affects the other vents as well. All the flaps may only be partially open. An initial bypass is to identify the leaking servos, disconnect the hose and plug both the hose and the nipple on the leaking servo so that the other servos can now operate normally. First to go is usually the middle one that controls the floor vents. It's actually a compound actuator with two pneumatic lines on it.

To see what I'm talking about, refer to the exploded schematics of the various air distribution systems at the end of my heater core replacement article in the 700/900 FAQ here
https://www.brickboard.com/FAQ/700-900/HeaterCoreReplacement.htm#Diagrams

The servos are unfortunately not repairable and the parts have long been unavailable from Volvo. They’re also a pain to remove, basically having to cut them out from the side of the box. If you do a bit of research there was a similar, but not identical, GM Delphi vacuum motor that could be retrofit to the 7090/900 air distribution box. I'm not sure if it's still available. It's long been on my do-list to try finding them again as both my 940s have failed servos and they're getting worse this past year.

In the past I've seen a number of good articles on dealing with, bypassing and trying to replace these servos. For starters, there's a feature article in the 700/900 FAQ here.
https://www.brickboard.com/FAQ/700-900/HeatingVacuumServoReplacement.htm#google_vignette
Others can be found elsewhere on line.

Please post back to let us know how you make out.

If I trip over more articles in the next while, I'll try to remember to come back and post link in this thread.

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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now