Two years sounds about right to me (chemist) with some variation allowed for storage conditions. Two years in my MB is about the maximum I allow also. Must work, my calipers have only had pads replaced for the last 25 years.

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/showthread.php?t=296386