Power Steering Fault

hi all can anyone here help me ??????????????????????

i have a 55 reg mondeo tdci 130bhp ghia and the easiest way i can describe it is that the steering wheel sticks in the central possition aeable nd knocked when parking or turning the wheel from side to side i have had it to my local garage and they told me that it was the rack and that the pump was leaking too so they changed them both however the knock has now gone but the sticking is still there the garage thought it was a faulty rack so fitted another but still no better it has gotten worse over the passed few weeks to the point its undriveable to the point that it is asif i have no power steering until i have turned the wheel nearly 360 degrees then a loud bang and its fine until the wheel is resting in the central position again then its back to square one i rang our local dealers and they told me they have had it befor but were unable to fix the fault and sold the guy a new car so that didn't fill me with confidance

please can someone help as i love the car and would love t have her back on the road