Big Red, I'm not sure, but did you ever check how much vacuum you were getting from the pump? I've had to rebuild one pump because it was only getting 15 In. Hg on the mityvacII. That solved all of my vacuum problems. Now my other car is not shutting off promptly, and has a brake pedal that loses vacuum every so often. It is only putting out 18 In. HG on the tester. I think 24 is normal. If you are not putting out enough vacuum the components will starve.

If you can't find any leaks, this seems likely to me.

I know that rebuilding my pump will solve this problem.

I also wanted to add that when I got my first 300SD, the blue/black check valve going to the door locks had been reversed by some crafty soul in an attempt to allow more vacuum to the other components. It worked for a bit.