Found a little more info. The GM replacement vacuum servos I was trying to remember are AC Delco Remy parts and among other fitments were apparently used in the likes of 1990s vintage Chevy S10 pickups. It appears they're also difficult to find now, also not cheap.

The part numbers to go looking for are:

o The single port vacuum servo is Delco p/n 15-71503 (GM p/n 01996701), a fairly straightforward replacement for the Volvo servo p/n 9463044 (superseded factory p/n 3522973)

o The two port compound servo (for floor/defrost) is Delco 15-71438 (GHM p/n 1996732), a more complicated replacement for the Volvo floor/defrost servo p/n 1388206 as the mounting is a bit different as well as the orientation of the servo connector.

Now here's something interesting I wasn't aware of before. I see in another Volvo forum where the leak may sometimes be the rubber seal on the servo shaft rather than a leaking diaphragm. It's a special shaped rubber o-ring seal and may not be easy to find or fashion a replacement. You still need to remove the servo to get at it, which is a rather involved process. My heater core article in the FAQ explains how to properly open up the center console dash area to remove the front of the air distribution box so you can get at the back of the servos to unbolt the threaded mounting studs and disconnect the servo rods. It involves many hours of work. To save time and frustration, others have simply cut the servos out of the side of the air box and mended the box later with something like JB Weld.

Refer to this article on OzVolvo for more info and pics
https://ozvolvo.org/d/5921-5921
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now