During your ownership of a vehicle, you will need to replace front and rear pads and discs due to wear and tear. You can’t escape even if you only use your car infrequently because if you leave it relatively idle in the garage, rust will set in. A vehicle’s braking systems need friction to bring it to a stop; hydraulic pressure will push brake pads up against a cast iron disc or else brake shoes will be pressed against a cast iron drum.
Once a vehicle is decelerated, the load gets transferred to its front wheels so it is up to the front brakes to do the majority of the work when it comes to bringing the vehicle to a halt.
Drums Or Discs?
When you brake, a considerable amount of heat gets created and it must be dealt with quickly in order for the vehicle to operate efficiently. The design of disc brakes is improving and they are now becoming more efficient; their open design when compared to drums ensures that there is less of a chance of them overheating.
At once time, it was normal to use drum brakes on the front and rear but disc brakes began getting fitted to the front of vehicles as they became more powerful around half a century ago. These manufacturers kept drum brakes at the rear of their vehicles as a means of providing brake function but disc brakes became the #1 choice for the front of vehicles.
Once disc brakes began to get applied to a vehicle’s four wheels (this first happened with small sports cars and larger cars), manufacturers decided that a smaller drum brake needed to be added to the rear hubs’ centre. Its design has since been improved and most parking brakes operate by applying pads straight to the main discs which means another drum parking brake is no longer required.
When test driving a new car, look at the operation of the parking brake to find out if it is working on a disc. New cars may have parking brakes which operate electrically and these can take some time to get the hang of.
Although cast iron is a great brake component material, its main weakness is the fact it corrodes easily. However, since the front brakes perform the majority of the braking force, surface rust is easily cleaned due to the pads acting on the discs.
The level of braking effort is far less on the rear of a vehicle; this is especially the case if the car in question is small and light and this might not be enough to get rid of corrosion from a rear discs’ surface if the vehicle is only used on relatively short trips.
Generally speaking, corrosion is not an issue with rear drum brakes. Light corrosion can be cleaned off with heavy braking but it can get worse if left alone. The result is surface pitting which is okay if it doesn’t weaken the discs.
At one time, this was one of the reasons for an MOT test failure but after computerised MOT tests were introduced, it became apparent that many vehicles were failing the test due to ‘brake discs pitted’ despite the fact this wasn’t enough to cause the disc to weaken.
Vehicle and Operator Services Agency (VOSA) has changed the manual and you can now only be failed for discs on an MOT test if they are deemed to have been significantly weakened.
Your rear discs need to be taken care of if you don’t drive often because storage in a wet garage means lots of rust can set in due to the damp atmosphere. Surface corrosion is down to the type of use rather than a fit for purpose problem so it is not covered by warranty.
Ultimately, your front discs will become too thin after wearing for a long time. The discs need to be replaced at this point in pairs and you should also change your pads at this stage. There is now a minimum thickness standard for brake discs and once your discs reach this point, it is unsafe to drive any further without making the change.
The disc can change shape due to uneven cooling and heating and this can be spotted when there is a juddering through the pedal once you apply the brakes. Clearly, thinner discs are more likely to become distorted than thicker ones.
When you are driving downhill for a long period of time, try not to hold your vehicle back with the brakes as this places a lot of heat in the discs. It is best to use a lower gear so you rely on engine braking instead of the brakes.
There is a chance you will have a disc that is not fitted flat against the vehicle’s hub assembly if the hub isn’t properly prepared prior to the fitting. The edge of the disc will move in and out slightly and feels like brake distortion when the wheel goes round. This is called ‘run-out’.
Sometimes, brake wear occurs after less than 30,000 miles and in other cases it may only happen after 65,000 miles. Pad life depends on a number of factors including your driving style, type of use and vehicle model.
Heavy braking from high speed causes more wear than frequent braking and motorway slip roads are one of the worst offenders. Incidentally, heavy braking can also cause brake judder and disc warping.
Asbestos has long since been banned from new and replacement brake linings (banned in 1999) so the friction material has been changed and the result is less durable brake discs. The earliest non-asbestos pad materials wore down very quickly but modern materials are much better. You are almost certain to have to replace front pads and discs and likely to need a change in rear pads and discs during the lifetime of your vehicle.
When new, pads will be shiny and may need some time to settle in. Be extra careful during the first 50 miles as braking performance will be affected.
Improved design means brake squeal is less common now; it involves the build up of brake dust. While anti-squeal shims can be effective, they wear down. Applying special grease to the back of the pads is another effective anti-squeal device.
If you allow the friction material to completely wear away from the brake pad, its metal backing material will run on the disc and the result is called ‘scoring’. This also hampers brake performance.
If you hear a distressing metallic noise when you hit the brakes, this is the first clue. A sticking piston in the calliper or lack of servicing could be the cause. The piston should release once you take your foot off the pedal and if it doesn’t, the pads stay in contact with the discs and wear down very quickly.
By ignoring these symptoms and continuing to drive, you will completely ruin the discs and they will have to be replaced. Make sure the pistons are retracting in the right manner once you change the pads.
It is hygroscopic and absorbs water from the atmosphere; this happens even if you don’t use the car. Flexible rubber hoses are the site of the water absorption. The fact that you can’t compress a liquid is the principle behind hydraulic brakes.
If you brake heavily, an example would be a long drive downhill where your brakes will get very hot and fill up with brake fluid. This fluid can even boil and vaporise. Although it is true that you can’t compress a liquid, it is possible to compress a vapour and if this happens, the brake will have a ‘spongy’ feel and this compromises brake performance. You need to replace your brake fluid every two years no matter how many miles you travel.