I had recently many of the same symptoms you report -- intermittent hard brake pedal, hard shifts, no engine shut-off, etc.. Because it was intermittent, I had a hard time getting a vacuum tester in there in time to "catch it in the act", but the problem finally progressed enough to where I measured just 5" vacuum at the pump. The spec I think is 22" for my '84 300D. I ended up putting a new pump in ($$$$$), and everything is back to normal. I kind of wish I had saved some money by rebuilding the pump I had. But it was a busy week, and I just couldn't find the time.