If your pads were metal to metal, this has been going on for some time at least. Your post about the car pulling to the right is less than a month ago I believe, so the rear brake issue has to be several months in the works. First thing first: get those rotors and pads replaced ASAP. Never rule out both rear calipers sticking at the same time, there is nothing that says that it can't happen, so make sure you test both calipers. Not sure how you would test the circuits/brake pressure, but what you can do is get a helper to assist you and get the 2 fronts off the ground and have someone apply the brakes while you try to rotate by hand, do that with front and rear.This will at least tell you if there is pressure on the fronts, though it may not tell you much else. Also check the condition of brake fluid. 24k miles ago puts you at close to 2 years (unless you're like me, that puts you at about 6 months :-). Hopefully this is linked to your pulling to the right because you've went from that, to cracked sub frame bushings, to a failed switch over valve....Hopefully you can get these issues sorted out and start enjoying your s124. Keep us posted.

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1990 190E 3.0L