I know this is an old post but I just came across this and I'm about to upgrade my brakes. I got the some 1st gen w126 calipers and rotors. My rear brakes have new rotors and freshly rebuilt calipers. I always liked that w126's have a softer brake pedal (more assist) and there is a larger volume of vacuum if you have to pump the brakes on a slick road without the pedal getting hard. But I installed a new master cylinder a year ago and don't really want to just ditch it. Can I use my w123 master cylinder on the w126 booster?

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect