How Do Brakes Work?

Ellie Dyer-Brown, 9 days ago

5 min read

  • How it works
  • Brakes
Close up of car brakes.

WhoCanFixMyCar explains how you car brakes work.

You rely on brakes for safety and control whenever you get in your car. Most drivers know this process only in its simplest form: you push down a pedal, and your vehicle begins to slow. However, this simple motion relies on a complex system of hydraulics and friction. If you want to learn more about brakes, this guide is for you.

Guide Contents:

How do brakes work in a nutshell?

Key components

Brake hydraulics

Power assisted brakes

Drum brakes

The handbrake


How do brakes work in a nutshell?

Pushing the brake pedal.

When you press the brake pedal, it forces hydraulic fluid through the braking system to the calliper at each wheel, which squeezes the brake pad against the surface of the rotating disc. When these parts meet, it creates friction, slowing the vehicle down.

Unfortunately, brakes don't last forever - this guide explains the average lifespan of different components. And, in case you've noticed any unusual symptoms from your car's brakes, here are five brake problems you shouldn't ignore.


Key components

Brake disc diagram

Brake discs

Brake discs sit between the wheel and hub, providing a surface for the pads to act against. They can come in two forms: vented (two discs joined together) and solid. Usually, vented discs are used at the front end of the car because the braking forces are higher.

Brake pads

Car brakes replacement.

Each brake disc is fitted with two pads at either side. These pads have to be extremely durable because they are designed to provide a high amount of friction against the disc.

Find out how much it costs to replace brake pads.

Brake drums

Drums are more common on older cars, though they still appear at the rear of some older, smaller and lower-powered vehicles. Brake pads are housed within the drum, so when you press the brake, the pads are forced towards the outer edge of the drum, slowing the car. 

Brake callipers

Callipers house the brake pads and pistons. They work similarly to clamps, deploying hydraulically actuated pistons to push the pads into the discs when you press the pedal.

The number of pistons a calliper has will affect the distribution of the braking force - the more there are, the bigger the brake pad can be, and the bigger the brake pad is, the better the stopping power.

This guide explains how to tell when your brake callipers need replacing.

Brake servo

A brake servo sits between the brake pedal and brakes, working with the master cylinder to increase the force deployed by the pedal. This can happen via a hydraulic pump or a vacuum. Essentially, the servo drastically reduces the amount of effort required from you when you apply the brakes.

Master cylinder

The master cylinder converts the power exerted by your foot on the pedal into hydraulic pressure. The movement of the pedal pushes pistons within the cylinder, which acts on the brake fluid, moving it around the system. 

Brake pipes/hoses

A series of thin pipes connect all the different brake components to transport brake fluid around the system. Most of these pipes will be made of copper, though in the places where they meet callipers, they need to be flexible, so you’ll often find rubber flexi-hoses.

ABS pump

ABS warning light

When a wheel ‘locks up’ during braking, the anti-lock braking system sensors will detect this and cause the brakes to pulse quickly. The ABS pump controls the distribution of brake fluid

The ABS and traction control systems are closely linked.


How do brake hydraulics work?

brake-fluid-leak

In hydraulic brake circuits, fluid-filled master and slave cylinders are connected by pipes. Here’s how it works:

  • When you push the brake pedal, it depresses a piston that’s located in the master cylinder, forcing fluid along the pipe.

  • There are slave cylinders at the wheel - the fluid travels to each of these, filling them and forcing the pistons out to apply the brakes.

  • The combined surface area of all the slave pistons is significantly greater than that of the piston in the master cylinder. This means that the brakes can exert a large amount of force. 

If you’re struggling to picture how this works, it’s similar to how a long-handled lever can easily lift a heavy object at a short distance.

It’s reassuring to know that most modern cars are fitted with twin hydraulic circuits consisting of two master cylinders in case one fails. 

Learn how to check and change brake fluid.


How do power assisted brakes work?

A brake booster/servo

The effort needed to apply the brakes is significantly less in cars fitted with power-assisted brakes. This is where the brake servo comes in. 

Most brake servos operate via a vacuum created in the servo's main body by a pipe running to the engine's air inlet manifold. Pressing the brake pedal pushes a rod into the housing containing an air filter and two springs. Air floods one side of the servo, which is separated into two sections (or chambers) by a diaphragm.

The vacuum in the side connected to the air inlet or hydraulic pump creates a pressure difference between the two servo chambers. In turn, this pressure difference pulls the diaphragm towards the master cylinder, forcing the pushrod into it (aided by a spring). As a result, braking becomes much easier for the driver.


How do drum brakes work?

A rear drum brake.

A drum brake system consists of brake shoes, a drum and hydraulic wheel cylinders. When you push the brake pedal, the two curved brake shoes are forced against the inner surface of a rotating brake drum by hydraulic wheel cylinders. The shoes are lined with a material that produces friction between solid surfaces to control or stop movement. Thus, when they are pressed against the drum, the friction slows the car.


Handbrake

Pulling a car handbrake up.

All cars have a handbrake, which usually acts on the two rear wheels. In the event of hydraulic brake failure, the handbrake system will only offer limited assistance; its primary function is as a parking brake. 

The lever inside the car pulls a cable linked to the brakes by a set of smaller levers, pulleys and guides. A ratchet on the handbrake lever keeps the brake on when it has been applied. 

Signs of a failing handbrake include:

  • The lever pulls up too easily.

  • The lever is too difficult to pull up.

  • The handbrake doesn't hold your car on a hill.

  • A warning light has appeared on your dashboard.

The presence of one or more of the above symptoms suggests you might need to book a handbrake repair.


If you think your brakes need repairing or replacing, you can use WhoCanFixMyCar to find reliable local mechanics.

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