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How Power Brakes Work
by Karim Nice

If you've ever opened the hood of your car, you've probably seen the brake booster. It's the round, black cannister located at the back of the engine compartment on the driver's side of the car.

Back in the day, when most cars had drum brakes, power brakes were not really necessary -- drum brakes naturally provide some of their own power assist. Since most cars today have disc brakes, at least on the front wheels, they need power brakes. Without this device, a lot of drivers would have very tired legs.

The brake booster uses vacuum from the engine to multiply the force that your foot applies to the master cylinder. In this edition of HowStuffWorks, we'll see what's inside the black cannister that provides power braking.

The Brakes!

The Vacuum Booster
The vacuum booster is a metal canister that contains a clever valve and a diaphragm. A rod going through the center of the canister connects to the master cylinder's piston on one side and to the pedal linkage on the other.

Another key part of the power brakes is the check valve.

The photo above shows the check valve, which is a one-way valve that only allows air to be sucked out of the master cylinder. If the engine is turned off, or if a leak forms in a vacuum hose, the check valve makes sure that air does not enter the vacuum booster. This is important because the vacuum booster has to be able to provide enough boost for a driver to make several stops in the event that the engine stops running -- you certainly don't want to lose brake function if you run out of gas on the highway. In the next section, we'll see how the booster works (and check out a cool animation!).

The Booster in Action
The vacuum booster is a very simple, elegant design. The device needs a vacuum source to operate. In gasoline-powered cars, the engine provides a vacuum suitable for the boosters. In fact, if you hook a hose to a certain part of an engine, you can suck some of the air out of a container, producing a partial vacuum. Because diesel engines don't produce a vacuum, diesel-powered vehicles must use a separate vacuum pump.

On cars with a vacuum booster, the brake pedal pushes a rod that passes through the booster into the master cylinder, actuating the master-cylinder piston. The engine creates a partial vacuum inside the vacuum booster on both sides of the diaphragm. When you hit the brake pedal, the rod cracks open a valve, allowing air to enter the booster on one side of the diaphragm while sealing off the vacuum. This increases pressure on that side of the diaphragm so that it helps to push the rod, which in turn pushes the piston in the master cylinder.

As the brake pedal is released, the valve seals off the outside air supply while reopening the vacuum valve. This restores vacuum to both sides of the diaphragm, allowing everything to return to its original position.

For lots more information, check out the links on the next page.

Lots More Information!

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