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Brake System Basics: What Every Counter Pro Should Know


2/1/2004
By Larry Carley

 

Brake parts account for a substantial portion of most parts store sales today, and should continue to do so in the future. Why? Because many parts in the brake system are high-wear items that have to be replaced every few years.

When you add up all of the things that can go wrong with brakes, it's a wonder there aren't more brake-related accidents. Fortunately, most people realize how important good brakes are for safe driving and usually have the brakes repaired when a problem occurs.

To better serve your customers, you need to be familiar with the basic components in the brake system, what they do, why they may need to be replaced and what kind of replacement parts to recommend for various kinds of driving. Let's start with the most commonly replaced parts and work our way down the list from there.

LININGS
Disc brake pads and drum brake shoes are typically replaced several times during a vehicle's life. Most vehicles still have disc brakes in the front and drums in the rear, but a growing number have disc brakes both front and rear. Either way, the front brakes work harder than the rears. Consequently, the front linings are usually replaced two or three times as often as the rear linings.

Brake pads and shoes are sold in matched axle sets (both fronts or both rears) and should be installed in matched sets on the vehicle. Many vehicles are very sensitive to the type of friction material in the linings so you should recommend a premium grade of replacement linings that are the same basic type (or better) of friction material as the OE linings to maintain the same feel, wear and stopping power.

There's a wide range of friction materials used in brake linings today including semi-metallic, ceramic, low-metallic and nonasbestos organic. What's more, many of the friction materials are "application engineered" for specific vehicle applications. Talk to your customer about how they drive their vehicle and follow your supplier's guidelines as to what type of linings would be best for their vehicle and driving needs.

 

Ceramic-based friction materials have become popular in recent years because of their stable braking characteristics, low-dust qualities and rotor-friendly nature. Ceramics are also quieter than semi-met and longer lived than most nonasbestos organic friction materials. If a vehicle was originally equipped with ceramic pads, the replacement pads should also be ceramic. Vehicles originally equipped with semi-metallic pads can also be upgraded to ceramic pads in many instances, especially if noise and/or rotor wear have been a problem. Severe-duty users should stick with semi-metallic pads because of their ability to withstand extreme braking temperatures.

ROTORS AND DRUMS
Rotors and drums may need to be resurfaced or replaced when the brakes are relined. Some minor grooving of the friction surface is normal, but if the surface is deeply grooved or heavily worn, resurfacing or replacement will be required.

Excessive runout, uneven wear and warpage are others problems that can afflict rotors. Variations in thickness can cause the brake pedal to pulsate, which may also be felt in the steering wheel. Resurfacing the rotors will make them flat again, but hard spots tend to come back after a few thousand miles. The best cure, therefore, is to replace the rotors.

Rotors and drums have wear limits for safety reasons. These parts must have a certain minimum thickness to maintain their integrity and to cool the brakes properly. If the metal is too thin, the rotor or drum may fail. Rotor thickness must be measured with a micrometer before a rotor is resurfaced or reused. The inside diameter of drums must be measured with a drum gauge. If the diameter exceeds specs, the drum must be replaced. Drums or rotors that are cracked or damaged must also be replaced.

Note: New drums and rotors come pre-finished from the factory and are ready to install. They do not need to be resurfaced to because doing so shortens the service life of the parts.

There are also two different types of rotors: cast and composite. Most rotors are one-piece castings made of iron. But some late-model vehicles have light-weight composite rotors with a cast-iron disc on a stamped steel center hat section. Composite rotors can be troublesome, but many experts say composite rotors should not be replaced with solid rotors. They advise to replace "same with same."

BRAKE HARDWARE
Drum hardware is another item that should be replaced when drum brakes are serviced. The return springs and holddown springs inside the drum are often severely corroded and weakened. Self-adjusters are often frozen with corrosion. Replacing these parts will restore the brakes to like-new condition and reduce the risk of future brake problems.

Disc brake hardware should also be replaced when the brakes are relined. Disc brakes don't have return springs but they do have bushings and slides that allow the caliper to center over the rotor. If these parts are worn or corroded, it may result in noise and/or uneven brake pad wear. Badly corroded or missing pad shims can also cause noise and brake drag.

Caliper slides and bushings should be lubricated with a moly-based, high-temperature brake grease, so make sure your customer gets a tube with the rest of his or her parts.

CALIPERS & WHEEL CYLINDERS
Calipers apply the brakes in a disc brake system, while wheel cylinders perform the same job in drum brakes. Both use pistons and hydraulic pressure to apply the brakes. The main problems here are fluid leaks due to seal wear or damage, and piston sticking due to corrosion.

Many experts recommend rebuilding or replacing calipers and wheel cylinders when the brakes are relined in high-mileage vehicles - even if the original parts are not leaking or sticking. Why? To reduce the risk of a comeback. Calipers and wheel cylinders are the lowest point in the hydraulic system and tend to collect moisture and sediment. As a result, internal corrosion often leaves pistons and bores pitted and rough.

Calipers are sold as bare units (new or remanufactured) or as "loaded" assemblies with new pads, shims and hardware. Loaded calipers provide everything your customer needs in one box and with one part number, which makes for easier installation.

MASTER CYLINDER
The master cylinder is the heart of the hydraulic system. It converts the force exerted on the brake pedal into hydraulic pressure to apply the brakes. A vacuum booster provides power assist on most vehicles. Inside the master cylinder is a pair of pistons (primary and secondary) in tandem that exert force against the fluid inside the unit. Worn or leaky seals, or bore wear or bore corrosion, can cause leaks and a loss of pressure. A classic symptom of a failing master cylinder is a brake pedal that slowly sinks to the floor at a stoplight. A leaky master cylinder should be replaced without delay!

BRAKE FLUID
Most vehicles use either DOT 3 or DOT 4 fluid, which is glycol-based and should therefore not be left open or spilled on paint. Brake fluid is "hygroscopic" and absorbs moisture. The fluid contains corrosion inhibitors to prevent rust, but over time the inhibitors break down as moisture contamination builds up.

Most brake experts recommend replacing the fluid when the brakes are relined or if the moisture content of the fluid is more than three percent. The condition of the fluid can be measured with a brake refractometer tool or chemical test strips. Moisture is bad because it causes a drop in the fluid's boiling temperature and increases the risk of pedal fade under hard braking. It also promotes corrosion.

BRAKE HOSES & LINES
Corrosion is the main issue with steel lines, while cracking and leaks may occur with old rubber hoses. The lines and hoses must withstand pressures that can range from a few hundred pounds per square inch up to almost 2,000 psi! Blowing a brake line is serious because loss of pressure results in brake failure!

Replacement hoses must be the same length as the original, with the same type of end fittings. A hose that is too long may rub against the suspension or other parts, while a hose that's too short may be stretched and damaged when turning or by excessive suspension travel.

Most hoses have a male fitting on one end and a female fitting on the other. If a hose fitting has a copper gasket, it should also be replaced when changing the hose.

Steel brake lines are made of special double-walled tubing because of the high pressure the line must hold. No other type of tubing should ever be used for a brake line (copper or aluminum tubing, for example). It's also important to replace the original brake line with one of the same size. Installing a larger or smaller diameter line may alter the brake balance in the affected circuit and upset the braking characteristics of the vehicle.

ABS COMPONENTS
The components in the antilock brake system are designed to last the life of the vehicle, but sometimes they don't. The most common problems are with wheel speed sensors (corrosion and electrical connections). On some vehicles, the wheel speed sensors are part of a sealed hub assembly that require replacing the entire hub if the sensor has failed.

ABS only comes into play when the road is slick or during a hard panic stop. ABS systems have self-diagnostic capability and will turn on the ABS warning light if a problem occurs. If the light remains on, it means the ABS system as deactivated itself and service is required. A technician must then read out the fault code(s) with a scan tool to diagnose the problem.

Other ABS parts that may need to be replaced if a failure has occurred include the ABS modulator unit or solenoids (which can be replaced separately on some systems but not all), the ABS pump, pump relay or pressure accumulator. All require specific service procedures and some require the use of a scan tool to bleed the system after installation.













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