Replaced 4 power steering pumps and two racks - still not working

Hello,
I am a transplant from Boston to San Diego.

I have a 2003 Cadillac Deville with about 75k miles on it. About 6 months ago, my power steering started to fail. I brought it to my mechanic and he replaced the power steering pump. About a month later the steering went again. They replaced the pump again under warranty and noted that the rack was not acting right. I had to go to another mechanic to get the rack replaced (because of cost). They used an aftermarket rack.

About a month later, the power steering went again. At my first mecanics request, I brought it to a cadillac dealer for testing/diagnosis. They claimed the pump was putting out proper pressure and the steering gear (rack) was not working. So, I had the rack replaced again - still a steering problem, and the pump replaced a fourth time.

The car now is worse than ever. The steering is almost completely gone. My original mechanic claims the aftermarket steering rack is the problem. Is that possible? They also tested the computer module and the signals are fine.

Any help is most appreciated.

Sell the car.

Anyone look at the steering column? If by going out, you mean that it is getting hard to turn the steering wheel, then it could be that the steering column is binding up and there is nothing wrong with the pump or the rack.

When the steering goes out, is it constant or intermittent? Also, did they check the ball joints and tie rod ends for binding?

And one more thing, how about the serpentine belt, is the tensioner for it OK?

Another one more thing, have the lines been checked for blockage?

Could a driver of this vehicle be habitually turning the wheel to full lock and holding it there for some reason?