I recently purchased a used 1999 ML430 with multiple issues. One of which is the steering will very hard to turn. I checked the fluid and it has plenty. So I jacked up the car and turned the wheel from side to side and it turns smoothly with no issues. Would this be a bad power steering pump ? The pump is not whining.

 

With out the eng. running, jack it up again and remove the reservoir cap and slowly turn the wheel from lock to lock. Have someone look into the reservoir and see if there are any bubbles. If so, continue turning lock to lock ( poss. 20-30 times).

If the bubbles remain you must replace the reservoir and not the pump.