See Figure 1
The hydraulic system is composed of the master cylinder and brake booster, the brake lines, the brake pressure differential valve(s), the wheel cylinders (drum brakes) and calipers (disc brakes).
The master cylinder serves as a brake fluid reservoir and (along with the booster) as a hydraulic pump. Brake fluid is stored in the two sections of the master cylinder. Each section corresponds to each part of the dual braking system. This tandem master cylinder is required by Federal law as a safety device.
When the brake pedal is depressed, it moves a piston mounted in the bottom of the master cylinder. The movement of this piston creates hydraulic pressure in the master cylinder. This pressure is carried to the wheel cylinders or the calipers by brake lines, passing through the pressure differential or proportioning valve.
When the hydraulic pressure reaches the wheels, after the pedal has been depressed, it enters the wheel cylinders or calipers. Here it comes into contact with a piston(s). The hydraulic pressure causes the piston(s) to move, which moves the brake shoes or pads (disc brakes), causing them to contact the drums or rotors (disc brakes). Friction between the brake shoes and the drums causes the vehicle to slow. There is a relationship between the amount of pressure that is applied to the brake peal and the amount of force which moves the brake shoes against the drums. Therefore, the harder the brake pedal is depressed, the quicker the vehicle will stop.
Since the hydraulic system is one which operates on fluids, air is a natural enemy of the brake system. Air in the hydraulic system retards the passage of hydraulic pressure from the master cylinder to the wheels. Anytime a hydraulic component below the master cylinder is opened or removed, the system must be bled of air to ensure proper operation. Air trapped in the hydraulic system can also cause the brake warning light to turn ON , even though the system has not failed. This is especially true after repairs have been performed on the system.