ALT-4 How Disc Brakes Work
Disk brakes are a fairly simple system. The disk referred to is also known as the rotor. The rotor is just behind the tire, and is round, with a smooth polished surface where the brake pads make contact on each side. The wheel studs that the tire mounts on are in the center of the disk, although it is usually a separate piece that the rotor can be slid off of.
As the tire turns, so does the rotor. Applying the brakes causes the pads to squeeze together. This results in friction, which takes energy from the cars motion, and ultimately becomes heat. As the rotor slows, so does the tire. The center of the rotor, when looked at edge on, has ribs inside to allow for ventilation of internal heat, and air flow helps reduce external heat. The friction causes small particles of the pad to wear off. This is the brake dust on your wheel. The heat, slowly over time, also causes the rotor to warp. This leads to faster wear of the pad. This is why rotors are "turned" or replaced. Turning removes a small portion of the contact surface, so cannot be done to often, so sometimes the rotor must be replaced. Allowing the pads to wear until the metal studs show causes scoring in the rotor. A warped or scored rotor is what causes a pulsation in the brake pedal.
Pads are generally made partly from metal, and partly from organic materials. Some are made with the inclusion of ceramic. Ceramic pads usually last longer, prevent brake fade, and result in less brake dust. This is because it is softer than metal. As a mechanic, I always use and recommend ceramic pads. The higher price is more than made up for by longevity, as well as safety. Most pads are equipped with a thin piece of metal that comes into contact with the rotor when the pads are close to being to worn off. This is called a warning tab. Not to be confused with brake squeal, this tab will cause a high pitched "singing" sound, often even when the pedal is not pushed, while the car is moving. Pads should be checked for replacement immediately in this case.
The brake pedal, when pushed, makes use of a lever to push a rod into the master cylinder. The master cylinder is filled with fluid heavier than water, that does not compress. Because it does not compress, it must go somewhere, in this case through the brake line to the caliper, which holds the brake pads in place. Often, vehicles with power brakes have a power booster that amplifies your foot power before pushing the rod into the master cylinder.
At the other end of the brake line is the caliper. The caliper holds the brake pads in place on each side of the rotor. It has the additional function of squeezing the pads onto the rotor. When the brake fluid is pressed through the lines, the fluid forces a piston on the inside half of the rotor to protrude, which is the source of pressure to squeeze the pads together. Letting off the pedal pulls the rod back out of the master cylinder (well, almost), which "sucks" the fluid back, pulling the piston back into the caliper, releasing pressure on the pads. The vibration from the contact points on the pads where they touch the caliper is what makes the sound known as brake "squeal". This is usually prevented by applying a small amount of silicone to these points to absorb the vibrations.
Thus, the brake system is a hydraulic system. Any air that enters into the system will cause a reduction in performance, if not all out failure. As the pads wear, there is more room in the caliper for fluid, which is noticed as low brake fluid, as the fluid that filled the master cylinder is now in the caliper. This is why you should not add fluid if no noticeable loss of braking is noticed. Checking pads and shoes is indicated instead. As the fluid gets lower, air can enter the system. A failing master cylinder also allows air into the lines. For this reason, the calipers are equipped with bleeder valves. By adding fluid to the master cylinder, pressing and holding the pedal, then opening the bleeder valve, fluid exits the system, carrying any air bubbles with it. This also serves to remove old fluid from the system, replacing it with the new fluid. Closing the bleeder valve before letting off the pedal keeps the system from sucking air back in. This should be done at every brake service.
That is about all there is to it. More technical information is obviously available, as well as specific procedures for performing service on your particular vehicle. www.autozone.com is a great site for US made vehicles.
