89 300SE

I changed the power steering fluid and filter for the first time since I bought the car. Before the change the power steering was quiet, now it makes a slight groaning sound as if it is low on fluid.

I used a Mann filter and MB power steering oil. I filled to the bottom of the minimum/maximum marks then took a drive, which is when I heard the sound. I checked the level and it was too high, so I sucked the resevoir dry again and refilled to the bottom of the minimum/maximum marks. It still groans slightly.

Any ideas of why it should be making noise after a fluid and filter change?

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire