PAS problem on a Mondeo Mk4 140 2.0TDCi Power assistance non-existent at low revs then suddenly cuts in. Whining noise on turning steering wheel seems to emanate from area of power steering pump. Refurbished pump fitted but after very short while seems worse than ever. If engine revs are increased then PAS seems much improved even when stationary. Fluid reservoir churns when engine running and when switched off a head of bubbles is seen on top of the fluid which takes some time to dissipate revealing thousands of pin prick sized bubbles still in the fluid. Any suggestions?