Sterring wheel very hard to turn while going slow or trying to park.

I have a 97 ford tarus the steering wheel is very hard to turn when going slow or trying to park but when the car goes at faster speeds the steering is fine. What could be causing this? The fluid was low so I added some with no change. I had the power steering unit replaced a year and a half ago could it be bad already? Or could it be just a hose or something like that. I did not notice a puddle under the car but the fluid was low. Any help you can give me I would appreciate. Thank you for your time.