Power steering pump screams
 

My 2001 Hyundai Coupe 2.0SE has a screaming power steering pump. When turning slowly it feels like the steering wheel is a 50p coin. I replaced it with a used pump from a 2000 Coupe and the scream is still there. I topped up the reservoir - it seemed to take only about 200ml. The belt is correctly tensioned. I've tried to bleed the system by taking to full lock (x20) with the car jacked up - no change. On full lock the pump belt screams and stops/judders - slowing the engine speed. What's going on?? The only solution I haven't explored is the possibility of a blockage (filter?) in the reservoir.