A question for your question: Guys is this the hose that goes through the Firewall and into the Intake manifold to create the Vacuum? I know I had a hose split right at intake manifold and it ran through the firewall. I believe it controls some of the Diaphrams for the heating.
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'75 Jeep CJ5 345Hp ChevyPwr and two motorcycles: it wasn't Volvos safety , it was Longevity that sold me

The recirc. button and its associated vaccum line must terminate to a vaccum actuator that controls the flapper door on the heater core, behind the blower motor housing.

I would suggest the area around the vaccum accumulator, the white plastic balls. The blower motor does not have to be removed to solve this problem, but it is a search and destroy mission without a vaccum diagram.

I have been in that area several times on my car (also a 1990) and have a comfortable familiarity with what goes on there. To really get the issue resolved I would suggest that you remove the glove box, center console kick panels, and knee bloster for maximum visibility. You may have to go further, almost like a blower motor r&r, to locate the faulty connection... if you are not intimidated. The problem is easy to solve just tedious.

The best solution is a vaccum diagram; unfortunately, Bentley lacks this detail. Someone here may have it, if not try your local Volvo dealer... if you get one I would like a copy.

Peace, Jon

Sometimes you can find a vaccuum leak in a area that you can not see by taking a piece of fuel line and place one end in your ear then use the other end like a Dr. would listen. You can move it around under the dash where you think the leak is and sometimes find it that way. Of corse the engine has to be running at the same time. Hope that helps locate your leak.
Dennis