How Brakes Are Applied and How the Hydraulic System Functions
A. Automotive brake systems fall into two major categories:
1. Service brakes (hydraulic brakes) and
2. Parking brakes.
a. Service brakes stop the vehicle when it is in motion.
b. Parking brakes hold the vehicle while it is parked. A parking brake is not designed to stop a moving vehicle.
B. Hydraulic brake systems
1. In modern vehicles, hydraulic systems transfer pressure applied to the brake pedal to brake shoes or pads. In some brake systems, pressure from the driver's pedal is enhanced by servo action and/or power boosters.
2. Most vehicles use two separate hydraulic systems to activate the brakes; therefore, failure of one hydraulic system will not result in complete brake loss.
a. On some vehicles, one hydraulic system activates the front wheel brakes while the other hydraulic system activates the rear wheel brakes.
b. On other vehicles, one hydraulic system activates the brakes on one front and rear wheel while another hydraulic system activates the brakes on the other front wheel and rear wheel. In this design, the brakes on one hydraulic system are always at opposite corners of the vehicle.
3. How the hydraulic braking system functions:
a. When the driver presses the brake pedal, hydraulic pressure builds in the master cylinder.
b. Hydraulic pressure travels through brake lines and valves to various brake activators, either wheel cylinders or calipers.
c. The wheel cylinders or capers convert the hydraulic pressure into mechanical force.
d. In drum brake systems, hydraulic pressure causes a wheel cylinder to press the brake shoe against the brake drum. In disc brake systems, hydraulic pressure causes a caliper to press a brake pad against a rotating disc. In both systems, therefore, the action of one component pressing against another creates friction and slows the vehicle.
e. When the brake is released, various devices move the brake shoes or pads away from the drums or discs.