When braking, the caliper uses a hydraulic piston to press the brake pads against the brake disc from both sides. The resulting friction converts motion into heat and slows the vehicle. In the process the softer pads wear away continuously, and over many thousands of kilometres the disc also becomes thinner and can develop grooves or a lip.
Typical signs that brakes are due are squealing or a metallic grinding noise, judder or vibration when braking (often warped discs), a longer stopping distance or the brake wear warning light. As a rule of thumb, brake pads last roughly 30,000 to 60,000 km depending on driving style, and discs often about twice as long. Highway drivers on flat terrain wear brakes more slowly than someone who brakes a lot in the mountains or in city traffic.
Pads and discs are replaced per axle (always left and right together) so the brake works evenly. If the discs are still within the minimum thickness, sometimes only the pads are swapped – but it is usually worth renewing discs and pads together. Replacement per axle takes about 1 to 2 hours, after which new pads must be gently bedded in.
As a guideline (no guarantee): replacing pads and discs on one axle costs roughly CHF 400–800 including parts and labour, slightly more on the rear axle with an electric parking brake. Changing pads only is cheaper (about CHF 200–400 per axle). For safety reasons, brakes should never be put off.