Here's how a brake booster and master cylinder work to stop your vehicle with the press of your brake pedal.
Stopping a heavy, 2000+ lb car is no easy task. It takes a lot of force to press up against the discs and drums on each wheel to stop a car quickly.
The hydraulics of brakes is quite simple. A primary piston (known as the master cylinder) pushes fluid into hydraulic lines that get fed out to the wheels. The piston inside the caliper (or wheel cylinder inside the drum) will expand with the fluid, causing it to glide up against the disc (or drum), slowing the wheel.
The brake booster was developed to sit in between the master cylinder and driver's pedal, to make it easier for it to press the pedal. While the master cylinder's diameter is already smaller than that of the caliper pistons, the force required to compress it is still great.
The brake booster works on the principle of vacuum differentials to aid in pushing the master cylinder. On one side, vacuum is sucked from the engine's intake. At idle, a valve in the diaphragm allows vacuum to be passed through the diaphragm, so that vacuum balances both sides.
When you depress the brakes, that valve moves, sealing off the vacuum side, while allowing filtered atmospheric air to enter the booster from the brake pedal side. This creates a pressure differential between the diaphragm, which helps to force the piston in the master cylinder to compress.
A giant return spring brings the diaphragm back to its rest position when the brake pedal is released.
The master cylinder consists of two small pistons in series. Each piston routes to two diagonally opposite wheels, for redundancy in case one springs a leak or the seal is compromised. Reserve brake fluid is contained in a reservoir above the master cylinder and is sucked into the piston assembly when the brake pedal is pushed.
Periodic brake flushes are required because brake fluid is hydroscopic and will absorb moisture and loose its effectiveness over time. Furthermore brake fluid will wear down with heat, and may become contaminated.