Sticking rear calipers will not make the car change direction, concentrate on the front.

When my left front caliper froze, the car pulled to the right. At high speeds, the right rotor turned red and the braking distance was terrible - only one brake working. At low speeds, no big problem as the rotor remained fairly cool. I did not notice much wear difference in the pads, but that is hard to detect from side to side.

I only found out by removing the left pads and finding the piston almost impossible to push back in, even with a C clamp. You should be able to push the piston in with a pair of channel locks and not much effort.

Do some more homework on your right caliper. Rebuilt calipers are not very expensive, even from Volvo, and it is safety first.

Klaus
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Volvos should be saved from the junk yards, not donated.