Very hard steering 2003 MB E320

Hi guys! I am a US Air Force major stationed in Venezuela for the time being. I have a 2003 Mercedes E320 (W211) and recently started having great difficulty with the steering. It has become unbearably hard to turn, even while moving. I replaced PS pump & valve and scanned using STAR tool but have yet to resolve the problem. Our embassy mechanic says it could be steering rack or a  PS hose which supposedly has a check valve that sometimes fails. Can you provide any advise regarding this problem? Is there a common steering ailment for this model car? Thanks in advance for any help provided! Major Loebe-out. 

1. Occurs when warm or cold

2. Occurs while driving as well as parked

3. Occurs always

4. Problem commenced about 3 months ago

5. No light or indicator on dash