Trooperbc, yep I have 4wheel ABS.

I too wonder why the engine has to be running. Kind of makes me wonder if people who step on the brakes with the engine off in everyday life are doing damage.

Then again I cannot exactly buy your hypothesis as to why you could damage anything with the engine running. Afterall that is how the vehicle is used 99.9999+% of the time.

Speculating, well at best brainstorming, more like guessing, maybe the rod itself would apply too much concentrated force against the diaphram housing if not helped along by a vacuum assist??? Let me try an analogy. Say you want to push on a car to move it. If you do it with one hand against the sheetmetal on a car on a slight dowhill the car moves and you don't dent the sheetmetal. This is like the rod (hand in the analogy) working with a vacuum assist (slight downhill in the analogy) with the engine running the rod doesn't put too much force on the diaphram. Alternatively, if you push on a car with one hand on a flat surface you might dent the sheetmetal. This is the rod working without a vacuum assist and it could damage the diaphram??? Maybe it is not even an ABS thing just a "new and improved" booster sytem??? Then again this is all really over my head.