I have checked the operation of the vacuum shutoff on my 82 300SD(617.951). I can pull 24" vacuum on it and it holds for hours. It develops quite a force. I know that the hooked end engages the control rod inside, and I can get it there pretty easily. What I can't do is get it to move the external linkage. I sure don't want the experience that Larry Bible so graciously shared about having the engine runaway. With the shutoff loose against the IP and vacuum on, I can still have it hooked on what I think must be the control rod inside and move that rod back against a spring force that it is held by. I can do the same with the shutoff removed and reaching my finger in and hooking it. It does not move the stop lever on the external linkage down against the valve cover. Should it really? The functional diagrams in the FSM show that the vacuum shutoff can pull back on the CONTROL ROD, but not necessarily all of the linkage I've read conflicting info on this. Can any of you shed some light on this?