Repairs - Brakes & Pads

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 deteriorates over time and because it happens gradually, many people do not 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.

Britalba  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.

Disc Brakes

A disc brake system consists of a brake disc, a brake caliper and brake pads. When the brake pedal is applied, pressurised hydraulic fluid squeezes the brake pad friction material against the surface of the rotating brake disc. The result of this contact produces friction which enables the vehicle to slow down or stop.

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 Mobil 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 Mobil Brake Fluid DOT 4 Standard Mobil Brake Fluid
230 260 155 160

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 do so before. Changing brake fluid when recommended prevents brake failure and maintains the boiling point at a safe level.