How Do Brake Systems Work in Subaru Vehicles? | How Does Subaru's Brake Assist System Work? | What Are Some Good Braking Tips?

All these questions and more are answered in today's lesson:

Technical Schmecnical Lesson 1: Subaru Brake Systems

Have you ever wondered how your braking system functions? Or perhaps you have never picked up that owner's manual and read it cover to cover to fully understand your vehicle. Well, wonder no more! We are here to present some of the more technical aspects of vehicles and their function and operation. Introducing our new blog series "Technical Schmecnical" will be an ongoing series and serves as a helpful guide and resource for vehicle owners to better understand their vehicles. All of our information comes from owners manuals, quick reference guides, technical manuals, and even our technicians themselves!

Braking Tips:

  • WARNING! Never rest your foot on the brake pedal while driving! This can cause dangerous overheating of the brakes and needless wear on the brake pads and linings.

  • When the Brakes Get Wet: When driving in the rain or after washing the vehicle, the brake pads may get wet. As a result, brake stopping distances will be longer. To dry the brakes, drive the vehicle at a safe speed while lightly depressing the brake pedal to heat up the brakes.

  • Use of Engine Braking: Remember to make use of engine braking in addition to foot braking. When descending a grade, if only the foot brake is used, the brakes may start working improperly because of brake fluid overheating, caused by overheated brake pads. To help prevent this, shift into a lower gear to get stronger engine braking.

  • Braking When a Tire is Punctured: Do not depress the brake pedal suddenly when a tire is punctured. This could cause a loss of control of the vehicle. Keep driving straight ahead while gradually reducing speed. Then slowly pull off the road to a safe place.

Subaru Brake System:

  • Two Separate Circuits: Your vehicle has two separate circuit brake systems. Each circuit works diagonally across the vehicle. If one circuit of the brake system should fail, the other half of the system still works. If one circuit fails, the brake pedal will go down much closer to the floor than usual and you will need to press it down much harder. And a much longer distance will be needed to stop the vehicle.

    • Brake Booster: The brake booster uses engine manifold vacuum to assist braking force. Do not turn off the engine while driving because that will turn off the brake booster, resulting in poor braking power.The brakes will continue to work even when the brake booster completely stops functioning. If this happens, however, you will have to depress the pedal much harder than normal and increase braking distance will increase.

    Brake Assist System:WARNING! Do not be overconfident about the brake assist. It is not a system that brings more braking ability to the vehicle beyond its braking capability. Always use the utmost care when driving regarding vehicle speed and safe distance.

    When you need to brake suddenly, continue depressing the brake pedal strongly to bring the effect of the brake assist.

    • Brake Assist is a driver assistance system. It assists the brake power when the driver cannot depress the brake strongly and the brake power is insufficient. Brake Assist generates the brake power according to the speed at which the driver depresses the brake pedal.
  • Note: When you depress the brake pedal strongly or suddenly, the following phenomena occur. However, even though these occur, they do not indicate any malfunctions, and the brake assist system is operating properly:

    • You might feel that the brake pedal is applied by lighter force and generates a greater braking force.

    • You might hear an ABS operating noise from the engine compartment.

Anti-Lock Brake System (ABS)

The ABS system prevents the lock-up of wheels which may occur during sudden braking or braking on slippery road surfaces. This helps prevent the loss of steering control and directional stability caused by wheel lock-up.

When the ABS system is operating, you may hear a chattering noise or feel a slight vibration in the brake pedal. This is normal when the ABS operates. The ABS system will not operate when the vehicle speed is below approximately 6 mph (10 km/h)

WARNING! Always use the utmost care in driving-overconfidence because you are driving a vehicle with the ABS system could easily lead to a serious accident!

Caution! The ABS system does not always decrease stopping distance. You should always maintain a safe following distance from other vehicles.

  • When driving on badly surfaced roads, gravel roads, icy roads, or over deep newly fallen snow, stopping distances may be longer for a vehicle with the ABS system than one without. When driving under these conditions, therefore, reduce your speed and leave ample distance from other vehicles.

    **Just because you drive a Subaru does not make you invincible! Use common sense, drive at a safe speed, and do not put others in harm's way by driving aggressively!

  • When tire chains are installed, stopping distances may be longer for a vehicle with the ABS system than one without. Be sure to reduce your speed and maintain a safe distance from the vehicle in front.

  • When you feel the ABS system operating, you should maintain constant brake pedal pressure. Do not pump the brake pedal since doing so may defeat the operation of the ABS system.

ABS System Self Check:

Just after the vehicle is started, you may feel on the brake pedal a vibration similar to when the ABS operates, and you may also hear the sound of the ABS working from the engine compartment. This is caused by an automatic functional test of the ABS system being carried out and does not indicate any abnormal condition.

Electronic Brake Force Distribution (EBD System):

  • The EBD system maximizes the effectiveness of the brakes by allowing the rear brakes to supply a greater proportion of the braking force. It functions by adjusting the distribution of braking force to the rear wheels in accordance with the vehicle's loading condition and speed.

  • The EBD system is an integral part of the ABS system and uses some of the ABS system's components to perform its function of optimizing the distribution of braking force. If any of the ABS components used by the EBD system malfunction, the EBD system also stops working.

  • When the EBD system is operating, you may hear a chattering noise or feel a slight vibration in the brake pedal. This is normal and does NOT indicate a malfunction.

What to Do if the EBD System Malfunctions:

  • If a malfunction occurs in the EBD system, the system stops working and the brake system warning light and ABS warning light illuminate simultaneously.

  • The EBD system may be malfunctioning if the brake system warning system light and the ABS warning light illuminate simultaneously while driving.

  • Even if the EBD system malfunctions, the conventional braking system will still function. However, the rear wheels will be more prone to locking when the brakes are applied harder than usual and the vehicle's motion may therefore become somewhat harder to control.

  • If the braking system warning light and ABS warning light illuminate simultaneously, take the following steps:

  1. Stop the vehicle in the nearest safe, flat location.

  2. Shut down the engine, apply the parking brake, and then restart it.

  3. Release the parking brake. If both warning lights turn off, the EBD system may be malfunctioning. Drive carefully to the nearest SUBARU dealer and have the system inspected.

  4. If both warning lights illuminate again and remain illuminated after the engine has been restarted, shut down the engine again, apply the parking brake, and check the brake fluid level.

  5. If the brake fluid level is not below the "MIN" mark, the EBD system may be malfunctioning. Drive carefully to the nearest SUBARU dealer and have the system inspected.

  6. If the brake fluid is below the "MIN" mark, DO NOT drive the vehicle! Instead, have the vehicle towed to the nearest SUBARU dealer for repair.


  • Driving with the brake system warning light illuminated is dangerous. This indicates your brake system may not be working properly. If the light remains illuminated, have the brakes inspected by a SUBARU dealer immediately.

  • If at all in doubt about whether the brakes are operating properly, do not drive the vehicle. Have your vehicle towed to the nearest SUBARU dealer for repair.

Brake Override System (BOS)

The brake override system (BOS) is an active safety feature first introduced on all 2012 Impreza 2.0i models and will be standard equipment on ALL 2013 Subaru vehicles, starting with the new 2013 BRZ. The BOS can help prevent claims of unintentional vehicle acceleration because it monitors the simultaneous application of both the brake pedal and the accelerator pedal by the driver while the vehicle is moving. Here are the key features of the system:

Brake Override System Features:

  • System Activation Delay-The system has built-in activation delay which senses the accidental depression of both the brake pedal and the accelerator pedal. This activation delay allows the driver to realize that both pedals are being depressed simultaneously. If both the brake and accelerator pedals are immediately released by the driver, the system will not activate. If both pedals are not immediately released, the system will activate.

  • System Activation-- The system activates when BOTH the brake pedal and accelerator pedal are applied. On initial activation, the system is time-delayed until the ABS control unit raises the brake fluid pressure to a specific level to control vehicle movement. Engine operation is reduced by the closing the throttle plate. Physical application of the accelerator pedal by the driver has virtually little or not effect on engine or vehicle speed.

  • Dual Stage Activation

    • Light brake pedal application-The system will take longer to activate when both the accelerator pedal is depressed and the brake pedal is lightly depressed. This delayed activation allows the driver to apply the brake briefly and still maintain accelerator pedal control.

    • Heavy brake pedal application -The system will immediately activate when both the accelerator and the brake pedal are depressed.

  • Deactivation-- The system deactivates when it senses only INDIVIDUAL operation of the brake pedal or the accelerator pedal. During deactivation, brake fluid pressure is released and the throttle plate is momentarily activated to maintain drivetrain pressure. The system positions the throttle plate above idle to operation to prevent the vehicle from lunging forward. The system then synchronizes the linear operation of the throttle plate and accelerator pedal for normal vehicle operation.

Read more about the Subaru Brake Override System here.

As always, please use caution and common sense while operating your vehicle. Do not become so overconfident in your vehicle abilities that you forget the fundamentals like keeping a safe distance from other vehicles, maintaining the speed limit, and paying attention to your vehicle, its warning lights, and please please please remember that servicing your vehicle every 3,000 miles is important to keep your vehicle running smoothly. Do not underestimate the importance of oil changes, state inspections, and vehicle safety checks, such as Stanley Subaru's 27-Point Inspection. If you have questions, please contact our Service Department by calling 800-439-8989 or by emailing our Service Manager.

Have a question or topic suggestion you would like us to cover? Let us know in the comments below and we will do the research and get it covered for you!

Have a great day and Happy Reading!

[Update: read more about the brake warning lights, what they mean, and what you have to do about them in our Dashboard Lights Deep Dive.]

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