Have you replaced the fluid? What type of fluid are you using? The correct fluid is ATF type F. Don’t use regular PS fluid. Synthetic ATF compatible with type F would be ideal if you can find it. Maybe Mobil 1 synthetic ATF??

Another thought. What’s the condition of the PS hose? If old, it could be deteriorating internally and messing up the pumps. Same thing with deteriorating steering rack seals. Debris might also end up in the pump. Correct, clean fluid is important. Especially after replacing the pump. Another option is is installing an in-line filter to protect the pump. Any sign of leaks near the PS hose or the rack?

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Will I buy another Volvo??? We'll see....