W140 Hard Steering
 

I have (2) W140's. A 93' 500SEL and a 94' S500. The 94' S500' steering is much harder to turn than the 93', so I figure something is wrong with the steering pump. This car has had the hard steering since I have had the car. I did not notice it was hard, until I drove my friends S420, and than bought the 93'. I just thought that is how it steered. My question is, do those pumps just stop working with no leaks, weird wheel vibrations or noises? I thought at one time, there was a small leak towards the front of the car, but it does not leak much anymore. The steering fluid level is normal.

Thanks