Counterperson Training: Brake Systems

There is no question that a counterperson’s ability to deal with brake inquiries, one of every parts store’s most important product categories, is central to his overall success in the position.

A professional counterperson must balance customer wants with their real needs, and the best place to start in this is to understand brake systems well enough to understand where problems can arise.

Brake systems have, for the most part, remained unchanged in most critical areas, despite the addition of control systems like Anti-lock Brake Systems and their sister technology, Traction Control.

Basic hydraulic brake systems employ a number of key components.

The Brake Booster is the first component to receive input from the brake pedal. Working in concert with the Master Cylinder, the Brake Booster amplifies the brake pedal pressure. There are four main types: Vacuum Boosters use the engine vacuum to create additional force; Hydro-Vac types combine vacuum boost and master cylinder functions into a single unit; Hydro-Boost types use the power steering system to create additional force; and Powermaster, which is General Motors’ self-contained, electrically driven unit.

The Master Cylinder is the starting point for the hydraulic system. When the brake pedal is pressed, the master cylinder creates hydraulic pressure. In order for it to function properly, the seals that prevent fluid from slipping past the piston must be in good working order, as must the surface of the cylinder bore. A scored bore or worn seals will result in fluid slipping past, causing a sinking brake pedal and reduced braking force.

The hydraulic pressure is transmitted to the other brake components through Steel Lines and Rubber Hoses. Two types of fittings are found on brake lines and hoses: SAE Double Flare and ISO-type metric. Size and diameter of tubing are important for proper system function and the fitting types are not interchangeable.

Hydraulic pressure is regulated by Valves that control hydraulic pressure application to the front and rear brakes under different stopping conditions. A Proportioning Valve is commonly used to reduce braking pressure to the rear wheels, reducing the chance of lockup under heavy braking.

Wheel Cylinders (sometimes called Slave Cylinders) are located inside the Drum Brakes. They convert the hydraulic pressure back to mechanical movement and work by spreading the brake shoes apart so they can press against the inside of the drum. Brake Shoes, as mentioned, are divided into primary and secondary. The forward-most brake shoe is the primary, the “trailing” shoe the secondary.

Calipers provide the same function as Wheel Cylinders, but are found on Disc Brakes. The Caliper Assembly includes the housing, piston(s), piston seal(s), dust boot(s), and mounting hardware. There are two types: fixed calipers, with pistons on either side of the rotor; and floating calipers, which press from one side only but “pull” the opposing pad onto the rotor.

Troubleshooting a malfunctioning caliper can be a difficult road for a technician, never mind the shade tree mechanic. They can often be the cause of high pad wear, brake noise, uneven braking, and other maladies that cause technicians to blame other components. While the advent of semi-loaded calipers (which include all hardware but not the friction) and loaded calipers has lessened this to a degree, many technicians persist in changing out other components rather than addressing caliper-related maintenance.

Inspecting a caliper includes checking the piston for leakage, the boot for damage, and the caliper for cracks. If a technician wants to overhaul the caliper, he needs to ensure that the piston bore is not heavily pitted or scored. If it is, then the caliper must be replaced. Of course, the technician will have invested a fair bit of time in discovering this, which is why the semi-loaded caliper has become more popular.

Brake Pads, like brake shoes, provide the stopping power when in contact with the rotor. A friction material is bonded or riveted to a backing plate. It is worthwhile noting that the Friction Materials Standards Institute Number assigned to a brake pad or shoe is only used to indicate a size and configuration, not quality or performance. It is simply a “standard” number to indicate that a pad or shoe will fit a certain application or applications, not how well it will perform. Likewise, edge codes do not indicate relative performance under real-world braking conditions.

Brake performance testing and certification has bowed in recently. In the absence of government mandates on performance in North America, two brake dyno-based performance validation procedures currently provide some assurance of performance. D3EA is the creation of independent engineers and testers, and is generally regarded as the most accurate simulation of whole-car testing done by automakers. BEEP was created by the Brake Manufacturers Council in consultation with the Society of Automotive Engineers, and is quite rigorous, though less all-encompassing and less expensive than D3EA. European specialists may also demand that friction materials meet the ECE Regulation 90 European brake safety test. This standard is a government mandate in Europe. Regulation 90-approved pads are indicated by the circled “E” mark on the pad backing, which confirms that the pads perform within 15% of the OE product.

Regardless of the brake testing certifications, brake pads can be formulated from one of a number of friction materials.

Friction formulations have become more specialized over the past few years, with terms such as application-specific, vehicle-specific, and application-engineered entering the lexicon. This is a result of more specific requirements at the OE level in terms of construction, stopping power, noise, pad life, fade resistance, dusting, etc. Generally, friction formulations designed for specific vehicles will be part of a premium line and provide greater assurance of performance across the aforementioned criteria. Using friction formulation types other than those the vehicle was designed for can provide increased performance in one area, but can also lead to poor performance in other areas. This may be acceptable to the customer–greater brake rotor wear may be traded off for heat resistance, needed when pulling a trailer for example–but this should be part of the discussion with the customer.

There are a number of basic categories of brake friction.

Ceramic brake friction is the most recent addition to the marketplace. Its prime selling points are low noise characteristics and near-invisible dust, which address many consumers’ concerns. They are excellent for most consumer use, but not for severe-duty, high-temperature applications.

Semi-metallic pads contain about 30% to 65% metal, which typically includes chopped steel wool or wire, iron powder, copper or graphite mixed with inorganic fillers, and friction modifiers that bond all the ingredients together. These pads are more durable and have excellent heat transfer, but also wear down rotors faster, have intrusive noise characteristics, and may not perform as well under low-temperature conditions. They are, however, well suited for higher-temperature applications.

Non-asbestos organic, sometimes referred to as organic or NAO, is made by mixing non-asbestos fibres such as glass, rubber, carbon, and Kevlar with filler materials and high-temperature resins. These pads are softer and create less noise, but they wear faster and create more dust.

Low-metallic NAO friction materials are a favourite of many European vehicle manufacturers and contain less metal than semi-metallic pads, about 10% to 30%, which helps with heat transfer. They stop well, but tend to wear quickly by North American standards, and are known to blacken the wheels of the cars that use them.

Disc Brake Rotors come in solid and vented designs. Vented designs dissipate heat better than solid designs. The heat generated under braking can in time cause rotors to warp and crack. Warped rotors can often be felt as a pulsating brake pedal on non-ABS systems. Cross-drilled rotors can be found for high performance racing applications but are generally unnecessary for street use.

Anti-lock Braking Systems (ABS) use essentially the same components as a conventional braking system, but add some high-tech hardware. An Electronic BrakeControl Module (EBCU) or an ECU monitors the signals generated by the Wheel Speed Sensors and the Exciter (or Tone) Ring located on each wheel in most systems (some use only one on the rear wheels). When the EBCU receives a signal that a wheel is about to lock up, it kicks the Modulator into action. This device is a solenoid-operated hydraulic control valve that takes over braking duties under impending wheel lockup conditions. It will apply and release pressure to individual hydraulic circuits (rear wheels share a single control circuit on some systems) in a way similar to pumping the brake pedal. The difference is that ABS will do this up to 20 times per second, and only on the wheel(s) that need it. Non-integral systems add the additional components separately; integral systems combine the EBCU, modulator, brake booster, and master cylinder into one unit.

Brake Fluid used in automotive systems comes in four types: DOT 3, DOT 4, DOT 5, and the relatively recently introduced DOT 5.1. DOT 3 and DOT 4, the most common types of brake fluids, are mineral-based; DOT 5 is silicone-based.

They are classified according to their boiling points, with DOT 4 having a higher point than DOT 3 and so forth. Both DOT 3 and DOT 4 are hygroscopic, which means they attract water. The greater their water content, the lower their boiling point. If it gets low enough, the fluid can boil during normal use, leading to brake failure. In addition to keeping a system clean, this is a key reason why brake fluid should be changed regularly (every four years).

Minimum boiling points for these specifications are as follows:

Boiling Point Ranges

Some confusion may arise from the addition of DOT 5.1 fluid to the brake fluid family. DOT 5.1 is not an evolution of DOT 5, as those accustomed to seeing new versions of software might believe. Rather, DOT 5.1 is a high temperature mineral-based brake fluid.

While generally unnecessary for most drivers, DOT 5.1 is suitable for high performance and high temperature applications in cars with ABS systems that cannot use silicone-based brake fluid.

Brake hardware must be in good condition to provide proper brake function.

Disc Brake Hardware includes anti-rattle clips and springs that secure pads in the caliper and prevent brake noise; guide pins on floating calipers that support and attach the caliper to the anchor plate; guide boots to protect them from corrosion; bushings and insulators that cushion caliper movements and help eliminate brake noise; and the caliper support key on some floating calipers that is used to locate and support it.

Drum Brake Hardware includes shoe return springs that retract shoes from the drum; hold-down springs that hold shoes on the backing plate; hold-down pins that hold the shoe in place on the backing plate; and the automatic adjuster, which compensates for friction material and drum wear.

There are many other items that are part of a braking system, and all parts must be in good working condition for a brake system to function properly and safely.

A counterperson must always have the requirements of safe, reliable brake performance in mind when discussing brake components and service with a customer.