I have to agree. I've only had my GS for about 5 weeks and the back brake is very weak compared to the one on my old bike. Not sure if this is common or not. However I must say that the front brake is loads better than my old bike. So loose on one gain on the other. Have seen a GS skid the back wheel, so I guess with a bit of force, they do work well. This was a planned skid (mate showing off)..
I'm thinking about adjusting the screw by the brake pedal, so the point when the brakes engage is higher up. I don't seem to be able to push the pedal far enough down to get the required braking.. Not sure if this a good thing to do or will help.. I'm no expert...
Not much help, only agreement. sorry.
Si