In my experience, the new VRs come with brushes that are 1/2 inch long. I know that may not sound like a sceintific statistic to use, but it does give one an idea how badly worn down yours are.
With a "cold" engine at idle, what reading do you get with a voltage ohm meter connected to the battery?
Mine reads 14.25-14.30, but I have a 100amp alt.(Bosch AL166X vs. the std. AL66X which is 80 amp).
You can get a new set of German brushes or should I say a new VR from this or any other MB WEB parts site for not a whole lot of $$$. You may have a worn commutator ring that could have accelerated the wear of your not-so-old brushes. See what stevebfl had to say about that in the following thread:
http://www.peachparts.com/shopforum/showthread.php?t=12213&page=1&pp=15&highlight=commutator+ring