Lesson 10: The Brake System

Brakes offer a way to slow the momentum of a vehicle for better control and safety on the road. A brake system uses hydraulic advantage to apply force to a friction material that generates heat. A moving vehicle has kinetic energy. Brakes convert kinetic energy into heat energy.

There are two types of brakes: disc brakes and drum brakes. The main differences are the shapes of the brakes and their friction material. Friction material provides the buffer that prevents metal from grinding metal when the brakes are applied. It can withstand the heat that is generated, although it wears out over time and has to be replaced many times throughout the life of the vehicle.

Disc brakes have a rotor (disc) for each wheel and brake pads are the friction material. A brake caliper applies great force to a brake pad that presses against the rotor when it is applied. This generates heat and stops the vehicle.

Drum brakes are shaped like a drum and brake shoes are the friction material. A wheel cylinder applies pressure to a brake shoe with a friction lining. Think of a circle within a circle. A hydraulic device expands the shape of the shoes to press against the drum. This generates heat and stops the vehicle.

The master cylinder is a hydraulic cylinder that is used to apply force to the brake calipers and/or wheel cylinders. The wheel cylinders (used in drum brakes) and the brake calipers (used in disc brakes) are both “hydraulic slave cylinders”. The master tells the slaves what to do. (This terminology is standard for the industry and is not intended to offend any particular culture which might have been affected by slavery historically).

Anti-lock brakes have complex electrical and hydraulic components which are used to avoid skidding wheels in a panic stop or in a poor traction condition. The anti-lock brakes (ABS) use signals from each wheel speed sensor to determine if one wheel is stopping faster than any of the others. If this occurs, the ABS control module (a computer) will signal the ABS hydraulic control unit to isolate that wheel, release brake pressure, and then pulsates the brake pressure to maintain equal speeds of all four wheels. This helps avoid skidding and maintains maneuverability during a panic stop.

How to Check the Brake System:

The brake system is often checked by the mechanic during routine oil changes and maintenance. Yet, it can also be checked at home. Consult the service manual (different from the owner’s manual) of the specific make and model for checking the brakes and finding the locations of each part of the brake system. Follow these steps to check the brake system:

• Start under the hood.
• Check the brake master cylinder for leaks.
• Check the level and condition of the brake fluid.
• Inspect all lines and the anti-lock brake module for leaks or loose, damaged connectors.
• Raise the vehicle and support it securely on a lift or jack stands (review Lesson 1 if needed)
• Remove all four wheels
• If the vehicle has drum brakes, then remove the drums.
• Inspect all brake lines and hoses for signs of leaking or cracking
• Inspect the caliper hardware and dust boots for damage
• Inspect the caliper (or wheel cylinders) for brake fluid leaking.
• Inspect the rotors for smoothness. If a rotor has any pits or grooves then it will have to be reconditioned or replaced.
• Check the thickness of all the brake pads.

If there is any variation in the brake pad (or shoe) thickness between the driver side and passenger side, then there may be an adjustment problem, a defective caliper or other defective hardware. Usually the front brakes wear faster since the front brakes are responsible for about 80% of the stopping. There is no blanket measurement for brake pad thickness since the thickness of new pads can vary by make and model of the vehicle. A few millimeters could appear brand new in one car but be an indication for pad replacement in another. If the thickness of the pad is 20% of the original thickness or less, then the pads should be replaced. The original thickness can be found in the service manual for the vehicle.

Lesson 10: The Brake System is one of twelve lessons that are also available as a free ebook download which is complete with illustrations. These lessons are based on the outline for the Boy Scout Automotive Maintenance merit badge and adapted by Crawford’s Auto Repair for a general audience. This article is for informational purposes only and the author does not assume responsibility or liability for any accident that may occur when working on motor vehicles. By reading this page you accept the condition that you are ultimately responsible for your own actions. For additional information see Brake System Services and Free Brake Check.

Copyright © 2014, Jeff Crawford. Permission is granted to republish this article for personal or commercial use as long as the content, citation, and copyright notice all remain intact and unchanged. There must be an active “follow” link to CrawfordsAutoService.com.

Lesson 10: The Brake System is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.