1993 Ford Falcon Steering problem

I have a problem with my 93 EB 4L sedan with the steering, the steering wheel gets very stiff and lumpy when getting round towards full lock in either direction, the steering wheel is also stiff and lumpy when you first start the car for a couple of minutes. Aidditionally when you go full lock right hand down the power steering pump appears to stall momentarily. I thought the pump was the problem and changed it at the weekend but no change. Should also mention the power steering fluid had gone black. The way I see it there is only the pump, hoses and rack in the power steering system, so is it the rack? Would welcome any advice,
rgds
Rexton