Good question. In the case of my car, a 1982 300D-Turbo, the original brake master cylinder failed in 2006, with 69,000 miles on the car. I had it replaced by an independent shop.

The pricing between what a shop has to pay their supplier vs purchasing the part online can be substantial. I recently picked up a replacement Ate master cylinder to have on hand, as I'm now prepared to do it myself, when signs of failure begin to appear.

At present, I don't have any plans to replace the master cylinder. I do wonder a little if the rubber/seals in the replacement cylinder will degrade with it sitting on the shelf, but my guess is that 2-3 years will not make much difference. Maybe I'll replace it after it has been on the car for 20 years, but we'll see.

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Marshall Welch
Seattle, WA
1982 300D-T