I agree with autozen. The autoparts business has gane the way of Wally world, cheap offshore parts that don't last. Sure they offer a lifetime warranty, so what? You get what you pay for. Parts sold by any of the McParts are very poor quality. I would disagree about NAPA. The problem is that NAPA is great for vehicles/equipment made in the U.S.A. not European imports. NAPA just reboxes parts for imports. These can solmetimes be poor quality as well. All hydraulic brake parts have a finite shelf life. The mc from McParts could be several years old. I would try to find a new ATE brand master. There is no experience like stepping on the brakes and having the pedal go to the floor. A W115 250 has a common Mercedes mc, shouldn't be too hard to find. The dealer may surprise you. The price for a new MB master cyl may not be much higher than a reman from autozone.

Good luck, bro.