Power Steering not working

Power steering failed while driving and very loud whine coming from pump. Fluid in resovoir appeared to not be circulating. No leaks or kinks in lines. Had proper level of fluid in decent condition, serpentine belt in good condition, pulley in good shape. Replaced pump with remanufactured unit and cleaned/flushed resovoir, fluid appeared to be circulating afterwards but still loud whin and steering would only work when the engine was reved above idle. Whine would get louder as you reved. Bled air from system, problem persisted. Installed another remanufacted unit but same conditions. Performed system flush, noted slight metal flakes in fluid that came from the steering gear. Flushed several more times. Problem presists. What should be tested or replaced now?