The brief
The 316i had a screeching noise while braking. On a BMW that screech, with the pad wear warning likely up, means the rear pads have worn down to the wear sensor and the metal's making contact. He brought it in, which is the right call, that's the warning that comes before the grind. The pads are the wear item, and BMW runs a wear sensor in a rear pad that completes a circuit when the pad's worn down enough, lighting the dash. The discs the pads clamp wear too, thinning and scoring over their life. The 316i's rear pads had reached the sensor and the discs had reached the end of their life with them, so it wasn't just pads, it was rear pads, discs and a fresh wear sensor as an axle set.
The diagnosis
On the lift the rear brakes confirmed it: the pads worn to the sensor with little material left, the wear sensor ground through, and the discs scored and below minimum thickness. The calipers and slides were freed off and checked, they were fine. The fronts still had life. So it was a rear axle set: new genuine BMW-spec pads and discs on both sides with a new wear sensor, the electronic parking brake retracted and reset the proper way, because you don't fit fresh pads to a scored, undersized disc.
The work
Both rear discs and the worn pads were removed, the hubs cleaned up, and a new genuine BMW-spec set of rear discs and pads fitted with a new wear sensor, the calipers and slide pins cleaned and greased, the parking brake reset, every fastener torqued to spec. The brake pad warning was reset and the pads bedded in so they'd grip evenly from the start. A road test confirmed quiet, even, progressive braking with a firm pedal, the warning light off.
The outcome
Quiet brakes, a firm pedal, even bite front and rear, sharp progressive stopping, and the screech gone. The 316i went home stopping properly again. Rear pads run to the sensor turn into scored discs the longer they're left, so doing the rear pads, discs and sensor as an axle set put the braking back where it should be, the safety job you don't put off.
