How Car Brakes Work and How to Tell When They Go Bad

Learn how your brakes slow and stop your car, and how to tell if your brakes aren't working properly.

Brakes are your vehicle’s most important safety system. Under normal conditions, a driver exerts about 70 pounds of force on the brake pedal (about the same amount of effort it takes to bite into a steak) to bring thousands of pounds of moving metal and plastic to a quick and controlled stop. A flawless brake system is absolutely crucial to driver, passenger and pedestrian safety.

Read on to learn how a modern automotive brake system works and some of the most common brake system problems.

What Are the Different Types of Car Brakes?

  • All modern vehicles come with hydraulic brakes. Hydraulic brake systems pressurize and transfer brake fluid to the wheel brakes assemblies. The brake assemblies (disc or drum) use the pressurized fluid to apply the brake friction material that generates the friction needed for braking.
  • Anti-lock brakes use wheel speed sensors, a computer control module and an electro-hydraulic actuator to prevent hydraulic brake systems from locking up during hard braking.
  • Working in conjunction with hydraulic brakes, hybrid vehicles feature a regenerative braking system that uses the electric driveline to initially slow a car. The hydraulic brakes ultimately bring the vehicle to a stop.
  • Parking brakes, independent of the hydraulic or regenerative braking system, are used to apply rear brake assemblies.

What Are the Main Parts of the Hydraulic Braking System?

Master cylinder

When you press the brake pedal, the brake linkage (rod) pushes on the master cylinder’s internal piston seals, pressurizing and forcing brake fluid into the brake lines, brake calipers and wheel cylinders. This causes the brake pads to press inward against the rotors and force the brake shoes to press outward against brake drums.

Power brake booster

Brake boosters assist drivers by multiplying the force they apply to the brake pedal. These boosters use engine vacuum, an electric pump (diesel engines produce little or no engine vacuum) or hydraulic pressure (usually the pressure of the power steering pump). This increases braking performance while decreasing pedal effort, making driving safer and more pleasurable.

Brake lines and hoses

Made of double-walled steel and multi-layer rubber and synthetic compounds, brake lines and hoses transfer pressurized fluid from the master cylinder to the brake wheel assemblies.

Disc brake assemblies

A disc brake assembly consists of a brake caliper, brake pads, disc rotor and mounting hardware.

Drum brake assemblies

A drum brake assembly consists of a backing plate, wheel cylinder, brake shoes/linings, hold-down and pull-back springs, brake drum and an automatic self-brake adjusting mechanism.

Brake switches

Modern brake systems include a brake warning lamp switch that tells the driver if the master cylinder is low on brake fluid, or there is a problem with the brake system. There is also a parking brake warning light that lets you know if the parking brake is engaged.

Parking brake

Applying the parking brake mechanically locks the rear brake assemblies to keep a car stationary when parked on a hill. It also helps stop a vehicle due to a hydraulic system failure.

A hybrid’s hydraulic brakes includes all the same parts as a standard hydraulic brake system.

How Does the Braking System Work?

Hydraulic brake systems convert mechanical energy (the spinning wheels) into heat energy by transforming and amplifying the force exerted on the brake pedal. The brake pedal acts like a lever. Along with the brake booster, it greatly multiplies the force applied on the brake fluid in the master cylinder.

The master cylinder also increases brake fluid pressure, then sends pressurized brake fluid, via brake lines and hoses, to the brake calipers and wheel cylinders. The pressurized fluid acts on the caliper piston, squeezing the disc brake pads inward, clamping the pads against the brake rotors.

On drum brakes the wheel cylinders slide outward, forcing the brake shoes against the drum using a wedging, jamming action — similar to how a bicycle coaster brake works. The energy of the pads clamping against the rotors and brake shoes pushing against drums generates friction and heat. This heat-friction, along with friction generated between the tire and road surface, slows rotor and axle (and wheel) rotation and ultimately brings the car to a stop.

On a regenerative braking system, stepping off the accelerator or pushing on the brake pedal causes a hybrid car’s electric drive motor to run backward. The drive motor spinning backward places a drag on the wheels and slowing the car, while also producing electricity that recharges a hybrid’s high-voltage batteries. While regenerative braking provides practically all the initial stopping power, the hydraulic system engages during panic stops or hard stopping at high speeds.

Signs the Braking System Isn’t Working Properly

Grinding (metal-to-metal), noisy brakes

  • Worn or damaged brake pads or shoes (possibly digging into and scoring the disc rotors or brake drums), or brake pad wear indicator rubbing against the rotor.
  • Worn, rusted, missing or broken brake hardware, excess brake dust accumulation, rust build-up on rotors or drums, or a stone, rust or some other foreign object jammed between a rotor and pad.

Steering wheel or brake pedal vibrating when stopping

  • Excess rotor “run-out” (thickness varies across the face of the rotor) or drums are “out-of-round,” badly rusted rotors or drums, contaminated brake linings, cracked, damaged or glazed rotors or drums.

Spongy or fading brake pedal

  • Low brake fluid, air trapped in the brake fluid or a leak in the hydraulic system.

Low brake, excess pedal travel

  • Brake self-adjusting mechanism not working, low brake pads or shoe linings, misadjusted master cylinder push rod.

Pulling to one side when stopping

  • Seized or leaking brake calipers or wheel cylinders, contaminated brake linings, faulty self-brake adjusting mechanism, build-up of excess brake dust or a front-end alignment out of adjustment.

Dragging brakes

  • Weak, damaged or rusted brake hardware, seized brake calipers or wheel cylinders, collapsed brake hose, misadjusted parking brake cables or misadjusted master cylinder push rod.

Grabbing or locking brakes

  • Sticking or binding brake hardware, failing or failed anti-lock wheel speed sensors, brake lining friction surfaces contaminated by grease or brake fluid or bad brake booster.

Excessive brake pedal effort

  • Seized brake calipers or wheel cylinders, clogged or collapsed brake hoses, contaminated brake linings or defective master cylinder, or power brake booster vacuum, mechanical or electrical problems.

A regenerative braking system has a unique pedal feel but can exhibit all of these same symptoms.

Brakes are one of your car’s most essential safety systems. Never dismiss a brake warning light or brake system problems. Disregarding brake issues can be dangerous and lead to much more expensive repairs. When it comes to your car’s brakes, it’s always better to be safe than sorry.

Bob Lacivita
Bob Lacivita is an award-winning ASE and General Motors auto technician, educator and freelance writer who has written about DYI car repairs and vehicle maintenance topics. His work has been featured in The Family Handyman, a Reader's Digest book and Classic Bike Rider magazine. He has been a career and technical educator for 25 years teaching automotive technology, as well as writing state, federal and organizational foundation grants. He also helped design a unique curriculum delivery model that integrates rigorous, relevant academic standards seamlessly into career and technical education.