You can expect to have to replace front discs and pads during a vehicle's life due to wear but are more likely to have to replace rear pads and discs because of corrosion. If you only use your car a little and always keep it in a garage rust is more likely to set in.

Braking systems rely on friction to bring the vehicle to a halt – hydraulic pressure pushes brake pads against a cast iron disc or brake shoes against the inside of a cast iron drum.

When a vehicle is decelerated, load is transferred to the front wheels – this means that the front brakes do most of the work in stopping the vehicle.

Considerable heat is created during braking and for effective operation this heat must be dissipated fairly quickly. Disc brakes are more efficient and their more open design compared to drums means they are much less susceptible to overheating.

When disc brakes were first fitted to all four wheels – initially to large cars and small sports models – a small drum brake was incorporated into the centre of the rear hubs for the parking brake.

As 'discs all round' has become commonplace, parking brake design has moved on and many now work by applying pads directly to the main discs – eliminating the need for a separate drum parking brake.

You might find it more difficult to apply and release a parking brake operating on a disc so it's important to check the operation of the parking brake when you test drive a new car.

Electrically operated parking brakes on newer cars may take a while to get used. The handbook will show a special release procedure to use if the car battery is flat.

With most of the braking force done by the front brakes, any surface rust is quickly cleaned off by the action of the pads on the discs.

Braking effort is much lower on the rear, especially on a small, light vehicle and may not be sufficient to clean corrosion from the surface of rear discs, particularly if the vehicle is used only infrequently and for local trips.

Corrosion is generally not a problem with rear drum brakes.

Initial, light corrosion can be cleaned off under reasonably heavy braking, but if left, this light corrosion gets worse and can lead to surface pitting which is acceptable as long as it does not seriously weaken the discs.
Surface pitting

This used to be a reason for MOT test failure but data available following the introduction of the computerised MOT showed that too many cars were failing the test on 'brake discs pitted' even though this was not sufficient to weaken the disc.

If you only use your car a little, you may need to pay the rear discs special consideration – putting a wet car away in the garage means it's going to spend lots of time in a damp atmosphere for rust to set in.

Front discs will wear and eventually become too thin. For safety reasons, vehicle manufacturers specify a minimum thickness for brake discs and when they reach this point, the discs must be replaced (discs should always be replaced in pairs). Pads must be renewed at the same time.
Distortion

Uneven heating and cooling can cause the disc to change shape and this can be detected as a juddering back through the pedal when the brakes are applied.

Thinner/worn discs are more likely to warp than newer, thicker discs.