Thanks Jeff... that may explain why the pedal can sometimes slip but then catch again.
The Check valve sounds interesting. I have not encountered that problem before. Therefore I'm not sure how to diagnose it. It seems to me that if it were a check valve then the vacume could not assist when foot pressure is placed on the brake pedal. Thus the pedal would remain high and not slip toward the floor. But then I'm only guessing about the function of the check valve.
1993 190E 2.3
2000 Toyota 4x4 Tundra