The braking system is the most important system in your car. If your brakes fail, the result can be disastrous. When you step on the brakes, you command a stopping force ten times as powerful as the force that puts the car in motion. The braking system can exert thousands of pounds of pressure on each of the four brakes. In modern systems, the master cylinder is power-assisted by the engine.
All newer cars have dual systems, with two wheels' brakes operated by each subsystem.
That way, if one subsystem fails, the other can provide reasonably adequate braking power. Safety systems like this make modern brakes more complex, but also much safer than earlier braking systems. Brake fluid, specially designed to work in extreme conditions, fills the system. In recent years, brakes have changed greatly in design. Disc brakes, used for years for front wheel applications, are fast replacing drum brakes on the rear wheels of modern cars.