Google works great!
http://home.howstuffworks.com/ac1.htm
It seperates the High side from the low side and supplies low temperature low pressure liquid refrigerant to the evaporator.
I don't know what Mercedes recommends. When I worked on my system I reused the old one.
It should not rust because there shouldn't be any moisture in the system. Moisture will freeze at the valve and clog it. That doesn't mean the valve is bad.
That is why it is so important to pull a vacuum for a long time. IMHO 1 hour isn't long enough. I let the vacuum sit overnight and hit it again in the next day. Provided the vacuum holds. If it doesn't you have a leak that needs to be fixed.
Danny