1999 Mercury Sable Power Steering Problem

Make sure the belt is not slipping.(Pump wheel stops, belt keeps goin)

Slipping belt CAN do that but if it is harder in one direction than the other, that suggests internal leakage in the rack and pinion assembly. The clue is the power assist will get better over a few minutes as the fluid warms up. Remanufactured rack assemblies have stainless steel liners installed to prevent that wear from taking place. The original housing was made from soft aluminum, and the teflon seals wear grooves into that aluminum. That's where the internal fluid leakage takes place.