The only brake issue that seems to consistently happen with the fairly simple M3W brakes is corrosion of the rear brake wheel cylinder and that cause lots of problems when it seizes. It may be a good starting point to check this out and confirm it is OK? If it is, put some grease under the dust boots to prevent problems in the future. Wheel cylinder is a common one, Motaquip VWC362 or QH BWC 3238.
