If the rear is lifting when you start the car up after it's been sitting for a while you've got a leak in the system. This causes the rear to settle down over time. The most common leak is the SLS valve.

The fluid can be black from being old, or from motor oil getting in through the pump. I'd think about doing a good flush with mineral oil and run the care for a bit and see if the level changes or if the fluid goes black again. If it stay's clear then you're good, if the level changes or it goes black again you've likely got a leak in the pump.

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