Power Steering Heavy at Low speed

Hi,
Having problems with the Power Steering at Low speed. I'm sure the system is supposed to be variable depending on road speed with low asistance at motorway speeds. Currently the effort to turn the wheel is constant making it a real pain to park. Does any of you clever people know how the system is controlled, adjusted or tested. Have checked the oil and belts and all seems ok.
Many Thanks. Ian

sounds like the rack and pinion unit going stiff or the pump is getting week on it, either one will make it steer hard.