In the past, when cars were only equipped with drum brakes, a brake booster was not necessary. The driver stepped on the brake pedal at his own discretion and the vehicle slowed down. If it pedaled harder, the wheels blocked. The introduction of disc brakes on cars led to the demand for a brake servo. A simple invention that actually consists of a round 'drum' with a membrane inside that divides that 'drum' into two halves. When the brake is not used, the pressure on both sides of the diaphragm is the same. If the brake pedal is depressed, a valve will open, leaving a vacuum on one side of the diaphragm and not on the other. The difference in pressure acts on a piston that increases the brake pressure.
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