I've been searching here for a long time but not found any issue described that's similar to this.
Problem started with a leak at the reservoir o-ring which I replaced and then bled the system following the proper procedure. After this is worked perfectly for several weeks. Then over the course of the next few weeks began to lose power assist as the car warmed up accompanied by pump whine (after ~5-10 minutes of driving). On first start while cold it works fine with no noise. There is no leak anywhere and it's not losing fluid. I've repeated the procedure to bleed the system a few more times just to be sure but no improvement. Is my pump toast?

 

If I'm reading correctly, you replaced the O-ring only, correct?

I strongly believe that the reservoir screen has a tear in it causing the whine.

Pay attention to proper bleeding procedure PDF.

 

I removed the reservoir and inspected it as best I could and the screen inside appeared intact.
Replaced power steering pump and that solved the problem.
The old pump was not smooth when I spun the pulley by hand and had loud clicking noise.