While I didn't have this particular problem, I had a power steering fluid leak and searches a couple of site for answers on where to check for the leak and how to repair the problem. One of the answers I found addressed your issue. You can search the forum at either www.mbworld.org
for the particular thread, but here is what I found and copied to a word file for my records as it relates to replacing the O-ring:
You have to drain the fluid from the reservoir first. I suck it out with a syringe until there was none. there is only one bolt that holds it very accessible, I think it was a 10mm. then you have to remove the return hose as well. then remove the clip that holds the reservoir to the p/s pump by wedging a screwdriver on the middle of the clip.to locate the clip very easy it is where the bottom part of the reservoir that connects it to the p/s pump. after you remove the clip slide out the reservoir locate the o-ring inside the bottom part of the reservoir where it connect to the p/s pump and replace it with the new o-ring. slide it in, replace the clip and the bolt as well as the return hose. fill it up with fluid and there start the engine until it is on operating temp and then turn your steering wheels left to right several times.
Again, I don't remember which site had the info or which member posted it, but a quick search on the sites I listed above should yield the original post. You should pay close attention to make sure no air is in the system. Turning the wheel from left to right several times after the fluid has been replaced should address this. Since this wasn't the problem I actually had, I can only pass on what I found, copied and kept for my records. I would search the other sites to get more detailed information.