Brakes taking a break?

Wear, corrosion, distortion and other common causes of failure

When you hit the brakes, load is transferred to the front wheels and the front brakes do most of the work to stop the vehicle. So it’s normal to have to replace front brake discs and pads during your car’s life because of wear – but rear pads and discs are more likely to fail because of corrosion.

Discs or drums?

Braking creates a lot of heat which has to be dispelled fast. The more open design of disc brakes makes them much less susceptible to overheating.

It’s normal to see discs at the front – providing most of the braking effort – and cheaper drum brakes on the rear to provide the parking brake function. Larger or more powerful cars tend to have disc brakes on all four corners.

Parking brake

Some cars with ‘discs all round’ will have been fitted with a small drum brake in the centre of the rear hubs for the parking brake, though most now work by applying pads directly to the main discs

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

Corrosion

• Cast iron is an ideal material for brake components, but it rusts easily.
• On the front, surface rust is quickly cleaned off by the action of the pads on the discs, but this may not be the case on the rear, especially on a small, light vehicle or one only used infrequently and for local trips.
• Corrosion isn’t normally 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. 
• Pitting used to be a reason for MOT test failure, but now discs should only fail the test if they’ve become 'seriously weakened'.
• Surface corrosion or pitting of discs isn’t a 'fitness for purpose' or warranty repair issue, as it mainly depends on how the vehicle is used and stored.

Wear

• Front discs wear and eventually become too thin.
• Vehicle manufacturers specify a minimum thickness, and when they reach this point the discs must be replaced (always in pairs).
• Pads must be renewed at the same time.

Distortion

• Uneven heating and cooling can change the shape of the disc.
• Distortion can be felt as a juddering back through the pedal.
• Thinner/worn discs are more likely to warp.
• Try to avoid holding the car back with the brakes on long downhill stretches as this puts a lot of heat into the discs. Use a lower gear and engine braking instead.

Run-out

If the hub isn't cleaned properly before a brake disc’s fitted, you can end up with a disc not fitted flat against the hub. This results in 'run-out' – as the wheel goes round, the edge of the disc moves in and out slightly, producing similar symptoms to brake distortion.

Pad wear

• There’s huge variation and no hard-and-fast rules.
• Some drivers can get 70,000 miles from a set of pads while others only 25,000 miles.
• The type of vehicle, type of use and your driving style all have an effect.
• Heavy braking from high speed causes most wear, with motorway slip roads being one of the main culprits.
• Brake discs seem to need changing more often, too. Asbestos was banned in brake friction materials from 1999 and replacement materials are harder.

Squeal

• Normally caused by a build-up of brake dust but not as common as it used to be, thanks to improved design.
• Anti-squeal shims fitted behind the pads can wear.
• Squeal can be reduced by applying a special grease (compatible with the rubber dust seals used in the caliper) to the back of the pads.

Scoring

• If your brake pads aren’t renewed in time then the metal backing material of the pad will run on the disc, causing scoring and seriously affecting brake performance.
• The first clue is often a distressing metallic noise when you hit the brake pedal.
• This is most likely to be simply because of a lack of care/servicing but can be caused by a sticking piston in a caliper – the part which pushes the pad against the disc.
• Discs are likely to need replacing too, particularly if early symptoms are ignored.

Brake Fluid

• Brake fluid absorbs water from the atmosphere and should be replaced every two years, regardless of mileage.
• If your brakes get hot they can heat up the brake fluid and in extreme cases, cause any water in the fluid to boil and vaporise.
• Though you can't compress a liquid, you can compress a vapour and if this occurs, the brake feel will become 'spongy' and full braking performance will be lost.