On my 97 K1500, "oreo", the steering gets incredibly stiff at low speeds when I'm trying to turn, like pulling into a parking space. Before I bought it, the truck had a whine from the pump, I checked it and had no fluid in the reservoir. The guy put some in there and the noise went away. Below ~5 MPH it's hard to turn the wheel. So should I flush the system or just replace the pump?