Autozone is going to charge you $150.00 for a Beck Arnley master cylinder with a 3 month warranty and it needs to be special ordered. Discount Auto Parts will order you a new Cardone cylinder for $65 with a lifetime warranty. Or you could simply click on the 'Buy Parts' link at the top of the page and have one sent to you from someone who specializes in Mercedes parts. Personally I'm more than a bit wary of the quality of parts you get at chain stores but I've installed many Cardone rebuilt components over the years and have had very good success with their products. If someone wants to sell you a private label Supr-Delux Quality Rebuilt unit then run away.

Don't neglect bleeding _all_ of the old fluid out of the system when you do this in order to avoid problems in the future.