How Emergency Brakes Work - Auto | HowStuffWorks

Using only levers and cables, each type of emergency brake is completely mechanical and bypasses the normal brake system. This ensures that a vehicle can be brought to a complete stop if there's a failure of the brake system [source: Ofria].

When you set the emergency brake, the brake cable passes through an intermediate lever, which increases the force of your pull, and then passes through an equalizer. At the U-shaped equalizer, the cable is split in two. The equalizer divides the force and sends it evenly across the two cables connected to the rear wheels [source: Owen].

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Motor vehicles use either drum brakes or disc brakes. Drum brakes are common in the rear wheels, while disc brakes are most common on the front wheels (or all four wheels). In a rear drum situation, the emergency brake cable runs directly to the brake shoes, bypassing the hydraulic brake system. In this simple, mechanical bypass, the emergency brake system requires no extra parts to control the brakes [source: Owen].

Cars with rear disc brakes have a more complicated emergency brake system, sometimes requiring an entire drum brake system to be mounted inside of the rear rotor, called an exclusive parking brake or auxiliary drum brake [source: Owen].

When the vehicle has rear disc brakes without an auxiliary drum brake, a caliper-actuated parking brake system is used. With this system, an additional lever and corkscrew is added to the existing caliper piston. When the emergency brake is pulled, the lever forces the corkscrew against caliper piston, and applies the brakes, again bypassing the hydraulic braking system. 

Electric e-brakes are available on some cars today. Instead of having a pedal, stick or center console lever, a small button on the dash signals an electric motor to pull the brake cable. Advanced electric brake systems utilize computer-controlled motors to engage the brake caliper [source: Zangari].

We'll discuss the importance of knowing when and how to use emergency brakes in the next section.

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