I would also make sure that your check valves are working properly - the vacuum reservoir should maintain the vacuum for a significant amount of time, even if you have a slight leak, unless the check valves are not working, which will allow the vacuum to bleed back towards the engine under heavy acceleration. I had two bad check valves in my '79 450SL, they were passing air in both directions. When I replaced them with new ones both my heater system and the door locking system worked much better.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C