HOW DISC BRAKE WORKS

About Brakes

Everyone knows that your braking system is what stops your car safely. At Kwik Fit we know how important the regular inspection and maintenance of your brakes can be. We have over 600 brake specialists nationwide who are fully trained and committed to maintaining your brakes in an expert and cost effective manner.

Today’s vehicle braking systems involve many different components working together to help you to stop and manoeuvre your vehicle in a controlled manner. The key components of your vehicle braking system include a master cylinder, servo, brake callipers, brake fluid and cylinders, disks, drums, pads and shoes. All the components are linked by a series of brake hoses and brake pipes containing brake fluid, so when you press your brake pedal the vehicle slows down and stops. The rate at which your brakes reduce the vehicle speed is determined by the amount and duration of foot pressure is applied to your brake pedal, and of course your braking system's condition.

Brakes are in use more often when driving around town than on a motorway journey, not forgetting that you may need to use your brakes to stop your vehicle quickly in an emergency. So maintaining an effective vehicle braking system is a top priority for every motorist.

Regular brake checks are an important part of any vehicle preventative maintenance plan. Brake checks can identify a problem before it begins to impact your braking efficiency, thus providing an opportunity to restore the effectiveness of your vehicle's braking system to its intended operating performance. The effects of a sudden brake failure can be devastating and in the event of an accident attributed to faulty brakes, the consequences of not maintaining an effective braking system could result in prosecution. Your vehicle will also fail its MoT test if braking efficiency is below specification.

The performance of your vehicle's braking system is tested on a rolling road to measure the amount of braking force generated while operating the hand and foot brake. The braking force generated is then recorded and compared to the manufacturer's specified tolerance level. Any reading or imbalance falling outside these tolerance levels will result in an MoT test failure.

As best practice for people who cover average or greater than average annual mileage and for normal coverage motorists, Kwik Fit recommend that a vehicle's braking system is checked at least twice a year.

We offer a free, no-obligation brake check that takes around 45 minutes. The inspection covers a full braking system inspection digitally measuring wear on your vehicle brake pads, shoes, discs and drums. We’ll always give you a full written report of our inspection showing the current condition of your braking system, together with a free no obligation quote should any work be required.

How can I tell if my brakes need attention?

There are a number of tell-tale signs to look out for when using your brakes. They include a grinding noise when you apply the brakes or a pull to the left or right. The grinding normally means that brake pads are excessively worn and need to be replaced along with the damaged brake discs. A pull to the left or right is normally indicative of a sticking or seized mechanical or hydraulic component.

You may also feel a continuous "pulsating" from the pedal or a "spongy" feeling. This spongy feeling indicates that there is air in the hydraulic system due to a brake fluid leak. The "pulsating" feel is normally associated with a distorted brake disc or drum.

More obviously, you may notice an illuminated brake warning light on the instrument panel or that your handbrake is pulling up higher than it normally does - in modern cars, more than 6 to 8 clicks.

For safety's sake, it's important to have your vehicle inspected as quickly as possible when you notice any of the above symptoms.

What do I need to know about brakes?

The purpose of a braking system is to slow and stop your vehicle. It is controlled by a foot (service brake) pedal and by a hand or parking brake.

The brakes also allow you to make an emergency stop if you need to.

It is therefore important that you ensure that your braking system is maintained and operating at optimum levels. It is a legal requirement that your braking system performs as intended and your brakes will be tested as part of your MoT.

Like all components, brakes and indeed braking performance deteriorate over time and because it happens gradually, many people don't notice the difference in their braking system. Having your brakes checked regularly could also save you time and money by preventing damage to other components.

How do brakes wear differently?

Different driving patterns have a dramatic effect on how often your brakes need servicing. For example, a set of brake pads could last up to 60,000 miles or more on a car driven mostly on the motorway, may last only 25,000 or 30,000 miles or less on the same vehicle driven in busy city traffic.

Front brakes normally wear out before rear brakes because they handle a higher percentage of the braking load, especially on front-wheel drive cars.

It is often recommended that brake pads should be replaced if the pad friction material has worn down to a thickness of 3 millimeters. Brake disc thickness should be measured  and replaced if they measure at or below the manufacturer's safe minimum thickness specification.

Kwik Fit also recommends that brake discs or drums are replaced in axle sets. Replacing one brake disc or drum could cause an imbalanced braking performance that could lead to further premature wear.

When new parts are fitted, it's vital to drive gently and carefully until they 'bed in', which takes approximately 200 miles. Excessive braking action on new parts will potentially damage them and lead to a loss of braking efficiency and performance.

Lifetime guarantee

To ensure your brakes are always at their best, we offer a unique brakes lifetime guarantee. Once you have pads or shoes fitted at Kwik Fit, the replacement of these parts when they wear out is absolutely free to you, as long as you own the car.

What happens when I push my brake pedal?

The brake pedal is connected to the master cylinder located within the engine compartment. The cylinder is filled with brake fluid.

When you push down the brake pedal, hydraulic pressure is created in the master cylinder, brake fluid is pressurised along a series of brake pipes and hoses to the hydraulically activated pistons in each wheel's hub assembly that force the friction material on your pads or shoes on to rotating parts, and that's what stops your car.

Drum brakes

A drum brake system consists of hydraulic wheel cylinders, brake shoes and a brake drum. When the brake pedal is applied the two curved brake shoes, which have a friction material lining, are forced by hydraulic wheel cylinders against the inner surface of a rotating brake drum. The result of this contact produces friction which enables the vehicle to slow down or stop.

ABS and parking brakes

The anti-lock braking system (ABS) works by limiting, applying and releasing the pressure to any wheel that decelerates too quickly. This allows maximum stopping force to be applied without the brakes locking-up and the car skidding.

The ABS tests itself each time the ignition is turned on. If a defect is detected for whatever reason, the ABS turns itself off and the normal braking system is used on its own. The ABS warning light will inform the driver of a defect in the system.

The parking or hand brake is a lever mechanism that is applied to hold a vehicle in a parked position. It activates braking components at the rear of the vehicle's braking system.

Why is brake fluid important?

Brake fluid plays a crucial role as it transfers the force created when a driver presses the brake pedal directly onto the wheel hub. Heat generated under braking, especially under heavier breaking or prolonged breaking, may affect the brake fluid which can only work if it is liquid and not so hot that it has become vapour. Therefore the boiling point your brake fluid achieves is critical to efficient braking as temperatures above the boiling point form vapour bubbles in the system, potentially resulting in brake failure.

In addition, the brake fluid serves as a lubricant of all movable parts and prevents corrosion. It has to be compatible with rubber seals and hoses thus allowing braking systems to achieve long service and optimal performance.

Why does brake fluid deteriorate?

A brake fluid has hygroscopic properties - meaning that it absorbs moisture during its life in your car through the pipes, hoses and joints that it lubricates. As the water content in the brake fluid increases, the temperature the liquid boils at decreases from when the brake fluid is ‘dry’ (ie no water content when new) to when it is ‘wet’ (contaminated with water).

The boiling point can be significantly reduced by water contamination below 5%. See below the difference in boiling temperatures for DOT 4 ESP Brake Fluid and how it exceeds the international standards for performance (known as the DOT standard).

Dry boiling point (°C)		Wet boiling point (°C)
DOT 4 Standard	DOT 4 ESP Brake Fluid	DOT 4 Standard	DOT 4 ESP Brake Fluid
230	266	155	171

Whilst regular servicing will check the boiling point of your brake fluid, vehicle manufacturers recommend that you change your brake fluid a maximum of every 2 years if not advised to before. Changing brake fluid when recommended prevents brake failure and maintains the boiling point at a safe level.