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Power brake units used on passenger cars are of four general types:
vacuum suspended; air suspended; hydraulic booster, and
electro-hydraulic booster. Most power brakes use vacuum suspended
units, which contains a large vacuum-powered booster device to
provide the added thrust to the typical power-brake. Pressure on the
brake pedal pushes forward a rod connected to the pistons of the two
master cylinders. The pistons begin forcing fluid into the front and
rear brake lines. At the same time, the brake-pedal pushrod positions
the vacuum-control valve so that it closes the vacuum port and seals
off the forward half of the booster unit. The engine vacuum line then
draws off the air, creating a low-pressure vacuum chamber.
Atmospheric pressure in the control chamber then pushes against the
diaphragm, dividing the two chambers. The pressure on the diaphragm,
which is locked to the pushrod, forces it forward, supplying even
more pressure on the pistons.
The safe driver is always ready to apply the total force needed to
stop their vehicle, even if the engine quits (removing the power
assist).
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