> To elaborate a bit, what it does is CUT fluid flow to the failed side, that's why it's relatively close to the master cylinder.
!! Are you SURE of that?! I'm not sure there's even a point to a feature like that -- if you've already lost a lot of fluid, chances are the lost-fluid side already won't brake. Second, if it's job is to plug the low-pressure side to prevent more fluid loss, then even if you then pumped the brakes the diff. press. sw. would then be blocking the line!
I've also never heard of such a thing, though that could simply mean I haven't been paying attention all this time.