its down in the reservoir - remove cap, remove the 10mm hex nut holding the plastic piece(scale) in place, remove plastic piece, remove spring, remove filter with needle nose pliers or wire, reverse procedure to button up.

As an added insurance, get yourself a new power steering reservoir cover seal.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)