It sounds like you are just low on P/S fluid. The reservoir is at the right front of the motor.

Get some fluid (MB# 000 989 88 03) if its low and then have someone check it for leaks. I have seen the shaft seal(between the two pump bodies) on the combination pumps (power steering/hydraulic system) fail and have the P/S fluid wind up in the hydraulic reservoir; so check it thoroughly.