Mondeo Mk4 Steering Issues

[Misteron]

Hi all, looking for some assistance with an ongoing steering problem. This alll started around 18 months ago, I had a problem with the steering pump, the steering was getting hard to turn, this was changed but made no difference, the pump was changed again but still no difference, the garage said there was nothing else they could do and refused to help any further. So, off to another garage, this time is was reccomended that I change the rack and the pump at the same time, otherwise the rack supplier would'nt warranty the rack, this work was completed along with a new resevoir, but allas, no change, in fact it was worse, while stationary there was no power steering, but when moving there was SOME assisted steering but with little "lock ups" whan turning a corner. The rack was changed again, the pump was changed again, the resevoir was changed again, still no fix. So, needed a new tyre, the chap who took the car in asked me what was the problem with the steering, I explained the situation and he replied that he was a Ford mechanic, and that to him it felt like there was an airlock in the system, so that weekend I jacked up the front of the car, took off the resevoir cap and turned the steering wheel from lock to lock for about 10 minutes, all the time I was turning the wheel little bubbles would appear in the resevoir and every now and then the resevoir would overflow and then setle down again, still with the little bubbles appearing, anyway, after the 10 minutes I started the engine and turned the steering from lock to lock again and BIG bubbles appeared, this happened a few times and then it settled down. After doing this exercise the steering did improve a little but still no pwere steering when static. As you can imagine, I'm getting a little frustrated and running out of ideas, so if any of you knowledgable chappies have any thoughts on this, please let me know. Oh, by the way, the pipes are all original and have not been changed!

Kind regards,

Ron.