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 vacuum booster. 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 the 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
Power Brake Diagram
Now let's put the parts together to see how power brakes work as a whole. This diagram provides both a closeup view and an example of where the brakes are located in your vehicle.
