Modern cars have a brake at each wheel applied by a pedal with which you may have some experience. When you press the pedal, it pushes a piston in the brake hydraulic system, which compresses the brake fluid. The fluid is forced through the brake lines to each of the wheels. The hydraulic pressure converts into braking action at each wheel. The brake lines are steel wherever possible, and rubber where they have to flex. The brakes at each wheel are one of two basic types – drum brakes and disc brakes.
Drum brakes have been around for a very long time. They work by pressing brake shoes against the inside of an open-ended cylinder attached to the wheel. Imagine something like the tins that Christmas cookies come in, straight-sided and flat-bottomed. The brake shoes press against the inside wall of the tin and the wheel is bolted concentrically to the flat bottom.
There are two curved shoes, each of which is about one-third of the circumference of the tin, or drum. They’re steel with a special friction material attached by either adhesive or rivets to the surface that contacts the drum.
When the brake is actuated, a small hydraulic cylinder, called the wheel cylinder, forces the shoes apart and into contact with the inner surface of the drum. Drum brakes used to be on all four wheels, but today, if they’re used at all, it’s on the rear wheels only.
Disc brakes work by squeezing a pair of brake pads against the surfaces of a brake rotor. Imagine an old-school vinyl record, with the wheel bolted concentrically to it. The brake pads, which are little steel plates with friction material attached to one side (as in the case of shoes, with either adhesive or rivets), ride on the record, or rotor as it’s called, one on Side A and one on Side B. The brake works by squeezing the rotor between the pads by means of a brake caliper, which is held against turning by a metal bracket. The caliper has one or more hydraulic cylinders built in that convert the pressure into a squeezing action.
Disc brakes are more expensive to engineer and build, but do a much better job of stopping, especially stopping in quick succession. One reason for this is the friction material covers a larger surface area than a drum brake. Another reason is that the disc brake is better at dissipating heat than a drum brake. It’s more exposed to the open air and has more overall surface area.
Thanks to the weight transfer that occurs during deceleration, the front brakes work harder than the rear brakes. Most vehicles also have more static weight on the front wheels. This is why drum brakes are no longer used on modern vehicles’ front wheels, and only rarely anymore even on the rear.
The parking brake usually acts only on the rear wheels, partly as a matter of practicality. The rear wheels don’t usually steer, and if they do, it’s only a tiny amount. It’s much easier to engineer a brake for them since these brakes won’t have to swivel back and forth. Plus, with no steering linkage, there’s more room for a parking brake.
The other reason parking brakes are usually on the rear can be discerned by the other name sometimes used for them – “emergency brakes”. If the main braking system fails, and you have to control the deceleration of the vehicle by the front wheels, steering control could be compromised. Parking brakes use a mechanical linkage to actuate them, so if there’s a hydraulic system failure they’ll still work.
Some parking brake systems use the service brake friction material, but many disc rear brake systems have a separate drum within the rotor that contains the parking brake.
Anti-lock brakes, or ABS, are almost ubiquitous today. Most of these systems use a sensor at each wheel to tell the computer how fast the wheel is turning. The computer uses this information to decrease fluid pressure to the brake at any wheel it thinks is locking up.
These systems monitor themselves for conditions that might prevent them from working properly. They turn on the “ABS” light in the dash if such a condition exists. When the light comes on, the ABS system is inactive, and the brakes will perform like standard non-ABS brakes – as long as the failure doesn’t prevent that. The electrically-operated mechanism can be sensitive to particulate and moisture contamination in the brake fluid.
What does it mean when someone says a car needs brakes? Usually, it means the friction material at one or more of the wheels is worn down close to the metal part of the shoe or pad. When the friction material is all worn off, and the metal backing presses against the drum or rotor, stopping power is drastically reduced, and damage to the drum or rotor is inevitable.
The Virginia State Safety Inspection procedure calls for failing the brakes if the friction material is thinner than 2/32 of an inch. But at that point, they’ll almost never last another year if the vehicle is driven at all. Since the State Inspection sticker is good for one year, I’m not sure that’s a great rule.
Sometimes, or even often, we recommend replacing the rotors or drums when the friction material is worn out. There are several reasons we might do this; sometimes more than one of these reasons apply. One reason that we would recommend replacing drums or rotors is that the surface has been damaged by the metal backing behind the friction material. This damage usually takes the form of scoring or grooves cut in the metal.
If the rotor is not replaced, the new pads will be damaged by it. Sometimes the old rotor or drum is damaged by heat; this ruins the part by warping it or causing hot spots or crazing (cracking) on the surface. This type of heat damage causes uneven braking and/or noisy brakes.
Sometimes the rotor or drum is just worn out and is below the discard level printed on it. And sometimes the drum or rotor is so eaten away by rust that we recommend replacing it. If one or more of these part failures are present, it makes sense to replace the drum or rotor while the brake shoes or pads are being replaced because of what’s called “overlapping labor”. In other words, we already have the brakes somewhat apart. So, to replace the drum or rotor while we’re at it is cheaper than doing the job separately.
Why Brakes Fail
Most of the brake repairs that we do involve friction material wear and drum or rotor replacement. Of the brake repairs that don’t, most of them involve the hydraulic system – the brake calipers, wheel cylinders, master cylinders (the one the pedal is attached to), or the brake lines and hoses. Sometimes there’s leakage, or maybe a wheel cylinder or caliper is sticking. Occasionally, the steel lines get so rusty that they burst and leak.
The majority of these failures could have been avoided, or at the very least, delayed by many years, by routinely flushing the brake hydraulic system to replace the old fluid and to rinse out the grit and goop that accumulate in the system.
Over time and use, the fluid deteriorates from heat and from absorbing moisture and other contaminants, and particulate contamination comes from wear in the moving parts in the cylinders and calipers. There’s a little more info on brake fluid in the fluids section.