According to the FSM vacuum diagram, it may be something awry with the check valve that feeds the manifold for all of the switch-over valves or possibly a restriction of some kind.
Here are a few links that might help you sort it out:
Mercedes Service Manual Source, CD/DVD/Paper
Mercedes-Benz Model 123 - Disc 2
http://www.startekinfo.com/StarTek/outside/12265/disc_2/program/Climate/83-604a.pdf