The rear brakes typically do little work, since most of the time the brakes are used to stop forward motion and others have described how the front brakes take most of the load when driving in the forward direction. It is not uncommon for the rear brake pads to outlast the fronts by several pad changes. This is good and bad. The good part is obvious - fewer pad changes. The bad part is that the rear brakes can suffer from corrosion of brake wear materials and the calipers that can freeze the pads in the pad channels of the calipers. This is normally an external to the caliper piston type of blockage that can be cleaned out once you get the pads out, using a screw driver and a wire brush and a hammer (the stuff gets firmly attached to the caliper and is very hard if it is a few years old). If you are getting less braking action when driving in reverse, the rear pad calipers are most likely frozen.

If left long enough, the caliper piston can end up being corroded and frozen to the caliper. This is harder to fix and usually requires new caliper seals. If that does not work it requires new calipers.

Good luck and I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)