I've got two '95 940 wagons with the same Girling front calipers. Both have Bendix boosters. Turbos have a vacuum pump and use a smaller booster, which I've never found provides as firm braking as the larger booster used with the NA engine that operates off manifold vacuum.

If the caliper slide pins are seized, rusted, damaged or badly worn then they are available and can be easily replaced. With enough effort banging them around with heavy pliers and a hammer, perhaps using penetrating oil and heat, reportedly they can usually be extracted. It's generally a sign that the caliper slides were not kept well lubricated during their life (with high temp silicone brake grease) and/or the rubber dust boots either dislodged or failed and allowed grit, salt and moisture to penetrate. I periodically do my own caliper rebuilds, replacing the caliper pin boots, piston dust boot and piston seals and have yet to need to go for a commercially rebuilt caliper in all the years I've had 700s and 900s. I've got a bunch of caliper pin spares I've accumulated over the years and generally try to swap out the most worn ones.

The caliper guide bushings are also significant wear points, with no replacements available. The caliper frames on both my cars are lingering minor problems I've yet to deal with. The upper bushings generally wear significantly more than the lower bushings (or is it the other way?), which I believe can make them more prone to locking up as seized calipers. I've only had that happen once, a hard turn in reverse out of a parking spot, and managed to unlock them by rocking the car back and forth in gear with the wheels turned hard one way then the other. They let go with a small bang. One thing I have tried to extend the life of the caliper frames is to swap them left/right (which may required swapping the pad plates up/down) to try averaging out the upper and lower bushing wear.

I've always been tempted to find a suitable machine shop that I could trust to do a good job replacing the bushings in the frames. It's not exactly rocket science, but braking is a critical operation and perhaps not the best place to start experimenting.

I've always been tempted to buy new caliper frames with new bushings to help cut down on caliper slop and chronic brake vibration, but they weren't cheap and I'm not sure if they're still available, or whether these are rebuilds and may not always have new bushings. I was once told here that the most practical option was to check a bunch of used frames in a yard (Pick & Pull) and wiggle one of your own pins in them to figure which have the least amount of wear.

I was not aware that 850 calipers will mount and could be used on the 940 front struts. The 1992-on 900s with ABS are listed as using the later Girling/Lucas Jumbo Type II vented front rotors. I don't know if those have the exact same offsets and dimensions as the rotors used on the 850/S70/V70s of that vintage, but I rather suspect not. I'd be interested to know more.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now