The brakes work on fluid pressure. When the pedal is depressed, it activates a plunger in the master cylinder. This pressurises the brake fluid in the pipes and hoses causing the brake components to move. That is why a brake system that has leaks or air bubbles will not operate correctly, or not at all. It is very important that the brake system is regularly inspected and the brake fluids changed. Disc brake systems are comprised of two pads per wheel, which clamp down on the brake disc when the brake pedal is depressed. When the fluid becomes pressurised, it sends a plunger in the caliper forward, forcing the brake pads on either side of the brake disc to squeeze it. This results in the slowing and stopping of the car. Drum brakes operate in the same theory as disc brakes, but with a different design. They both use friction to slow and stop the car, but drum brakes use shoes, instead of pads. The shoes are inside of the steel brake drum itself, which spins with the wheel. When brake pressure is applied, a component called the wheel cylinder expands, forcing the brake shoes apart and toward the inside edge of the drum.
All cars have a split braking system so that the pressure to the brakes are applied to the wheels on separate lines. This is done for several reasons, safety among them. There are two basic types of split braking systems. One is a front and rear split system. The second is called a diagonal split braking system. Front brakes on vehicles typically sustain about 70% of the braking load, which makes sense since vehicles are generally traveling forward. The rest of the 30% goes to the rear brakes. Whichever braking system is installed on your vehicle, you can find some comfort with anti-lock braking systems (ABS). These were developed to help unsafe stopping condition. It helps the driver keep the vehicle under control while it is brought to a stop, minimizing skidding and the loss of control that could easily lead to accidents.
Brake fluid is possibly the most neglected component of the braking system. Most drivers check their tyre pressures and change their engine oil at frequent intervals. But, when it comes to brake fluid, virtually no one ever changes it in their car. The function of the brake fluid is to provide hydraulic pressure when the driver puts pressure on the brake pedal. This is done through the master cylinder(s) to the calipers in order to clamp the brake pads against the brake discs.
There can be significant problems in how the brake fluid works. Overheated brake fluid can and will boil in the caliper. Boiling produces gas bubbles within any boiling fluid. Gas is compressible so boiling brake fluid leads to a soft brake pedal with long travel. In extreme cases, an overheated brake fluid may lead to the driver needing to pump the brake pedal several times in order to get the car to a complete stop.