I fixed a problem similar to your's on my 855. The rear would vibrate STRONGLY when braking. I thought i would have to replace the rotors and pads on the rear, but it turns out the rear pads were just glazed over. I think this happened when I tried to clean my rims while they were still hot. The cleaner solvant may hav caused brake dust to cake and adhere to the pad. Anyways it is a quick fix. Remove the pads and rub them around on some clean smooth concrete or use a rough sandpaper on a block to break the glaze. I also used 3m pads on a drill to clean up the rotor surfaces. upon replacing the pads i used anti-squeal/vibration goop on the back of the shims. They should be covered in a rubber material, but mine were worn. This was just my problem though, rotor warp can also cause bad vibes.