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What is a Power Brake?

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  • Written By: Matt Brady
  • Edited By: Jenn Walker
  • Last Modified Date: 13 September 2016
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    2003-2016
    Conjecture Corporation
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The power brake is a modern braking tool for cars that’s designed to make braking safer and easier. A power braking system works to exert far more force upon a set of brake pads or shoes than is exerted upon the brake pedal by the driver. Power brake systems come equipped with a master cylinder, which pumps brake fluid through a series of tubes connected to each wheel, and a power brake booster. With a working power brake system, one merely needs to tap on a brake pedal to exert considerable pressure on the brakes.

When a driver pushes down on the brake pedal, brake fluid is released from a holding tank into the master cylinder, which houses two pistons that compress the fluid and force it through a series of tubes linked to each wheel. The pressure created by the brake fluid is immense, acting with great force upon each wheel's set of pistons and calipers. When the fluid reaches the wheels, it causes calipers and pistons to exert force upon the braking apparatuses—called pads in the case of disk brakes, and shoes in drum brakes. Most cars use disk brakes in the front, which slow a tire down in much the same fashion as brakes on a bicycle, and drum brakes in the back.

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The power brake booster adds even more braking pressure. It sucks air in through a one-way vacuum, and uses that air to exert force upon the brakes when the brake pedal is depressed. The power brake booster also serves as a back-up power brake system in the event of engine failure. When an engine fails, the car is unable to pump brake fluid, thus eliminating power brakes. The power brake booster, however, keeps a reserve of air in such an event, giving the driver enough easy pumps of the brake pedal to come to a stop. If the booster's air supply is exhausted, the brakes are still functional but much harder to operate.

Power brakes require a very particular type of hydraulic oil to operate. Brake fluid has special properties that are able to withstand different environments as well as the pressure a power braking system imposes. Brake fluid is designed to boil only at extremely high temperatures, up to 460 degrees Fahrenheit (238 degrees Celsius), much higher than a car would create. It's also designed to keep its fluidity even at extremely low temperatures. Other types of fluid don't meet the particular needs of a braking system, and if used in place of brake fluid could jeopardize passengers by causing total and immediate brake failure.

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