Ok - I am somewhere handy with cars - but there's for sure more to learn. I've been fighting Power steering issues with my Volvo XC90 (2005) for some time now.
THE ISSUE: Steering wheel won't keep itself centered - pulls to either side - BUT not like wrong wheel alignment or trye pressure issue - but litery as force from power steering - and am saying that - because it acts exactly the same when car is turned on but standing still. It's not the steering rack - at least 99% sure it's not the rack - like it's not bleeding or losing liquid in any shape or form - it's dry as you can imagine. Now my mate suggested that he had similar issue in past and it turned out to be sticky vales within steering rack. As suggested - I tried to clean it using Trans Tune Sea Foam - didn't work - flushed the system with new liquid afterwards and feel no difference. Stering Pump makes no noice whatsoever - which I would expect if it was failing - however - I took it apart.
Pressure relief valve #1
Pressure relief valve
Pressure relief valve #3
The pressure relief valve is slightly damaged. See that's where my handy knowledge lacks expertiese - not too sure how the steering pump functions and if the relief valve is damaged - shouldn't the power steering be acting up when turning - which it's not - it only acts up when trying to keep straight. In a sense - when keeping it straight - suppose the pressure coming to steering rack is at it's lowest - compared to pressure required to turn the steering - Could really the pressure relief valve be cause of this?
Well it could be the pressure release valve. If not that it would have to be the rack itself, but the wreck accepts pressure from the pump through the pressure valves and if any of the valves go bad and open and close and open close by themselves, it will do that. Pray it's not the valves inside the rack
