The latest model of the servo used on the Mini and some other cars is a very dependable unit. But it still has rubber parts inside and these do wear out. Several things will tell you when it's too late to do preventative maintenance. If you are slowly loosing brake fluid or you see white smoke out of the tail pipe, then the main diaphragm is leaking. If your pedal pressure is too high for the stopping distance, then the main seals are bypassing. The kit includes both the main seals and the main diaphragm for the MKIII servo.