by Jim Kerr
Stopping a car takes a lot of power. A 200-horsepower engine may accelerate a car from zero to 100 kph in about eight seconds, yet the brake system can stop that same car from 100 kph in less than half that time! Our lives and the lives of others depend upon the proper operation of the brake system every time the brake pedal is pressed. Brake problems should be checked out as soon as possible. Here are some of the more common types of faults that may occur.
Two types of brakes are used on vehicles today: disc and drum. Each can have different types of problems. Disc brakes are found on the front of modern vehicles and are now often used on the back wheels as well. The disc is a cast iron plate (called the rotor) that is connected to the rotating hub and wheel assembly. When the brake pedal is pressed, brake fluid is forced from the master cylinder through tubes and hoses to the calipers mounted over the discs. The fluid pressure forces a piston out of the caliper and the piston pushes brake pads against the disc until the disc is clamped tightly. The harder you step on the pedal, the harder the disc is clamped or slowed.
Drum brakes, found on the rear of some vehicles, uses fluid pressure to force brake shoes outward into the rotating drum. The pressure of the shoes on the drum slows the wheel and hub assembly.
A pulsating brake pedal or vibration when the brakes are applied is the most common complaint. This can result from warped or distorted brake rotors or drums. As a warped disc rotates between the brake pads during a stop, it moves the pads back and forth. This motion is transferred into the fluid inside the brake caliper and then onto the master cylinder. You feel the fluid pressure change as a vibration on the brake pedal.
Brake discs can be warped by overheating the disc, a heavy impact, or incorrect torque on the wheel nuts. If pulsations are a problem, re-torque the wheel nuts (with a torque wrench!) first. This may correct brake pulsations in many vehicles without having to do any other work.
Things that can cause the brake rotor to overheat and warp include driving with your foot on the brake, long downhill mountainous grades without using lower gears, or a caliper or caliper piston sticking. The first two problems require a different driving technique, but sticking calipers and pistons require service work. The caliper may need replacing or overhauling. For many common vehicles, it is nearly the same cost to install rebuilt calipers as it is to buy the rebuild kits and overhaul them. Take your choice. Be sure to clean the caliper mounting surfaces. All sliding mount type calipers should have a coating of high temperature lubricant applied to the mount so they will slide freely.
Correcting a warped disc is done by machining it on a special brake lathe. The lathe cuts both sides of the disc at the same time to ensure the surfaces are parallel. Brake rotor surfaces must be parallel. It only takes a thickness variation of less than 1/2 the width of a hair to cause pedal pulsations. If the disc is worn too thin for machining, then it must be replaced. A thin disc cannot dissipate the tremendous heat generated during braking and it will soon fail or cause brake fade. The minimum thickness of a disc is cast into the disc by the manufacturers
or is listed in the shop manual.
Drum brakes can also warp, just like disc brakes. Setting the parking brake when the brakes are hot can quickly warp a drum. Let the brakes cool if you have been using them hard before applying the park brake. Re-machining or replacing the drum is necessary to correct for warpage.
Brake squeal is caused by the brake pads vibrating against their mounts. This can sometimes be corrected by tightening the mounting tabs on some styles of brake pads, or coating the back side of the pad with an anti-squeal coating. The coating doesn’t stop the pad from vibrating, but it acts as an insulator so there is no metal to metal contact. Some manufacturers have special ant-squeal shims that fit behind the pads to stop the noise.
Metallic pad material tends to squeal more than organic linings, but it is important to match the type of pad material with the brake rotor and vehicle. Using the wrong pad may not create enough friction and the vehicle stops poorly, or squeals at every stop. Some brake squeal is considered normal during light brake application, but a constant squeal from the brakes is a warning to have the brakes inspected as soon as possible. A constant squeal may be caused by a wear limit tab on the brake pad coming into contact with the rotor. Some manufacturers use this method to warn drivers the brake pads are nearly worn out.
Intermittent brake squeals can sometimes be corrected by “burnishing” the brakes. Burnishing is a process where the vehicle is driven to about 50 kph and then braked very hard to a stop. Let the brakes cool for a couple minutes and repeat the complete procedure about ten times. The burnishing process cleans the surface of the rotor, heats the surface of the pads to remove any glazed build-up, and transfers some of the pad material onto the surface of the rotor for higher friction between the pad and rotor. Obviously, for safety reasons, burnishing the brakes must be done where there is no other traffic.
Finally, if the red brake warning light on the dash comes on, have the brake system checked immediately. It may only be the parking brake applied slightly, but it could also indicate a loss of brake fluid. Partial or total brake failure could occur!