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Expect to replace front discs and pads during a vehicle's life due to wear and rears due to corrosion
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.
Drum brakes front and rear used to be the norm but as vehicles became more powerful in the 1960s more and more manufacturers began to fit disc brakes to the front – to provide most of the braking effort – while retaining drum brakes on the rear to provide the parking brake function.
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.
Cast iron is an ideal material for brake components, but can corrode easily.
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.
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.
VOSA(Vehicle and Operator Services Agency) has subsequently revised the testers' manual and now discs should only fail the test if they have become 'seriously weakened'.
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.
Surface corrosion or pitting of discs is not a 'fit for purpose' issue, nor a repair covered by warranty, rather it relates to type of use.
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.
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.
Try to avoid holding the car back with the brakes on long downhill runs as this will put a lot of heat into the discs. Use a lower gear so you make use of engine braking and use the brakes less.
If the hub isn't prepared properly before a disc is fitted then you can end up with a disc not fitted flat against the hub assembly. 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.
Brake wear is an indeterminate science – in some cases the brake pads will do 70,000 miles while other drivers will find they need changing after only 25,000 miles. The type of vehicle, type of use and your driving style all have an influence on pad life.
Although town driving involves frequent application of the brakes, such low speed use doesn't cause as much wear as heavy braking from high speed. Motorway slip roads are one of the main culprits and this type of heavy braking is more likely to contribute to disc warping and hence brake judder etc. too.
Brake discs seem to need changing more often too. The friction material of brakes has changed since asbestos was banned from new and replacement brake linings from October 1999 (this doesn't apply to pre-1972 vehicles).
Early non-asbestos pad materials were much harder and more aggressive in terms of disc wear but this seems to be less of a problem now.
Front discs and pads will be replaced during a vehicle's life due to wear, while rear pads and discs may have to be replaced due to corrosion.
New pads can be a bit shiny when new and need time to bed in. Braking performance will be affected for the first 50 miles or so – brakes will often feel less precise and driving style should take this into account.
Not as common-place as it used to be due to improved design, brake squeal is caused by a build up of brake dust.
Manufacturers fit anti-squeal shims behind the pads, but these can wear.
Squeal can be reduced by the application of special grease (compatible with the rubber dust seals used in the caliper) to the back of the pads.
If the brake pad friction material is allowed to wear away completely (it should at least be checked when the car is serviced) then the metal backing material of the pad will run on the disc and cause scoring. Brake performance will be seriously compromised too.
The first clue for the driver is often a distressing metallic noise when the brakes are applied.
This can result from a lack of servicing or be caused by a sticking piston in a caliper – the part which pushes the pad against the disc. If the piston doesn't release when you take your foot off the pedal the pads remain in contact with the disc and wear much more rapidly.
If you continue driving and ignore the symptoms, irreparable damage will be done to the discs and these will also need to be replaced.
When pads are renewed it's important to check that the pistons are retracting properly.
Brake fluid is hygroscopic and absorbs water from the atmosphere – even if the car isn't used. Most of the water absorption takes place through the flexible rubber hoses.
Hydraulic brakes rely on the principle that you can't compress a liquid.
Under heavy braking, such as a long downhill descent, the brakes get hot and heat up the brake fluid. In extreme cases, water in the brake fluid can boil and vaporise.
Though you can't compress a liquid, you can compress a vapour and if this occurs brake feel will become 'spongy' and full braking performance will be lost.
For safety brake fluid should be replaced every two years, regardless of mileage.
(29 November 2011)