The most common type of service required for brakes is changing the pads. Disc brake pads usually have a piece of metal on them called a wear indicator.
When enough of the friction material is worn away, the wear indicator will contact the disc and make a squealing sound. This means it is time for new brake pads.
There is also an inspection opening in the caliper so you can see how much friction material is left on your brake pads.
Sometimes, deep scores get worn into brake rotors. This can happen if a worn-out brake pad is left on the car for too long. Brake rotors can also warp; that is, lose their flatness. If this happens, the brakes may shudder or vibrate when you stop. Both of these problems can sometimes be fixed by refinishing (also called turning or machining) the rotors. Some material is removed from both sides of the rotors to restore the flat, smooth surface.
Refinishing is not required every time your brake shoes are replaced. You need it only if they are warped or badly scored. In fact, refinishing the rotors more often than is necessary will reduce their life. Because the process removes material, brake rotors get thinner every time they are refinished. All brake rotors have a specification for the minimum allowable thickness before they need to be replaced. This spec can be found in the shop manual for each vehicle.
For more information on disc brakes and related topics, check out the links on the next page.