More power steering problems

I was having problems with my power steering whining and eventually losing functionality. I tried many things including changing the fluid, changing types of fluid, and cleaning the screen as best as I could. I took it to my mechanic and he replaced the fluid, replaced the belt and adjusted it and it seemed to be better. Slowly the symptoms have returned and worsened to the point that I can no longer steer properly.
Usually it is worse when it is a cold start and it gets better after driving a bit. This has also disappeared after the week or two . This morning I went to see if I could adjust the belt some to see what effect this had. When I started it made no whine but after I turned the wheel slightly it began to whine. I had checked the fluid level before starting and it was fine. when the whine started I opened the reservoir and noticed that the fluid seemed frothy.
I am thinking the air in the fluid could be the problem, but why is there air tin the fluid? I am thinking this all goes back to the screen. The lines should be pressurized and I would think air could not enter through them. I am thinking the fluid cannot get through the screen and is being churned in the reservoir.
Any input would be helpful before I start buying parts or take it back to the mechanic.

 

To purge the air from the system you need to turn the steering wheel back and forth from fully turned to fully turned. You don't need the engine on. This is easier with the wheels jacked up. Check the fluid level after doing this to see if it dropped from air coming out to the top.

If the screen is clogged bad, then there's no good way to clean it. You could try a new one, or find a used one at a junkyard that is clean. You can take a picture with a flash down the top of the reservoir to see part of the screen to check if it is clean. The fluid level needs to be below the screen to see the screen. Or if you remove the reservoir you can shine light through one end and see if light shines through the center part of the screen. The center is probably representative of the unseen edges; either the whole screen is clogged or not clogged. A new o-ring is needed when reinstalling the reservoir.

I posted some pictures of a screen with the top of the reservoir removed in this thread. I'm not saying to remove the top. I'm just saying you can see what the screen (albeit clogged and broken) looks like from the pictures.

The screen is plastic, so it will break if you put anything hard or sharp down in the reservoir. If it does break and a clogged screen was the problem, then it would improve the symptoms, so you'd know that was the problem.

If the screen is clogged bad, it could be from the hoses deteriorating and bits of rubber clogging it.