Automotive Brake Operation and Safety
Since the first set of cart wheels hit the ground turning thousands of years ago, people have been perfecting ways to make sure they’re able to stop those wheels on command.
Of course, the answer was the humble brake, created to ensure that anything that rolled could stop in time to avoid an accident -- and stay stopped as needed. Unlike the simple hand brakes of earlier eras, today’s braking systems are sophisticated, multi-component systems.
Whether they’re bringing the family car to a safe stop or slowing down the big rigs that crisscross the nation, modern brakes are more reliable, last longer, stop better and keep people safer than ever.
So, let’s get this thing started and learn a little more about the brakes on today’s vehicles.
Automotive Brake Systems
Most automobiles and light trucks licensed for use on public roadways are equipped with both primary service brakes and emergency brakes, also known as parking brakes. Though all part of stopping and staying stopped, each is a separate system and serves a specific purpose.
Known as a “frictional” brake system, the service brake is the vehicle’s primary stopping system, mechanically separated from the emergency braking system. As the name implies, these brakes use friction to stop the vehicle, relying on pads that contract inwardly onto the outside surfaces of rotors with disk brake systems, or shoes that expand outwardly onto the inside surfaces of drums for drum brake systems.
Most commonly operated by fluid pressure in lighter non-commercial vehicles, service brakes work by employing a “bias,” a separation of braking duties in which most of the force is applied to the front brakes. This design allows the vehicle to remain in control under even the heaviest braking applications. Too much rear brake bias, and the vehicle will slide out of control under heavy braking conditions. The same thing will happen to a vehicle with too much front brake bias.
So while the safe, reliable stopping ability of our vehicle’s brakes is something granted, a properly working braking system is bound to need some preventative maintenance to ensure it remains in top working order. Replacing brake pads or brake shoes at the first hint of brake noise is important.
This is also the ideal time to inspect and service the disk rotors or brake drums. Remember, heat is the braking system’s greatest enemy. Brakes that have over-heated at some point often result in warped rotors on disk-brake vehicles, and egg-shaped, out-of-round drums on drum brake vehicles. In both instances, a noticeable pulse in the brake pedal when braking can result.
Any rotors or drums that have had their braking surfaces damaged by heat, the mounting rivets of the shoes or pads, or other causes should be replaced entirely. They’ll lessen braking performance and may even lead to shortened life for the shoes and pads you’ve just replaced. Consider new components, rather than mechanically resurfacing the old ones. Replacement costs little more and gets your vehicle back in service sooner.
When maintaining brakes, don’t forget about the brake fluid in hydraulic braking systems. Brake fluid attracts moisture, which can cause damage to the brake components as well as catastrophic brake failure. It also is subjected to extreme heat and can burn. Flush the brake lines and replace the brake fluid according to the manufacturer's recommended time table. Replace the fluid at the first sign of water or if the fluid smells burnt.
Emergency brakes are a secondary braking system installed in motor vehicles, required by both federal and state laws. Also known as e-brakes, hand brakes and parking brakes, emergency brakes are independent of the primary service brakes that are used to slow and stop vehicles.
The emergency brake generally bypasses the normal fluid brake system using only levers and cables. This ensures that a vehicle can be brought to a complete stop if there's a failure of the brake system.
When the emergency brake is set, the brake cable passes through an intermediate lever which increases the force of the pull, and then passes through an equalizer. At the U-shaped equalizer, the cable is split in two, which divides the force and sends it evenly across the two cables connected to the rear wheels.
In modern, non-commercial vehicles, there are four types of emergency brakes:
● Stick lever, which is generally found under the instrument panel (in older-model vehicles)
● Center lever, which is found in between separated front seats
● Pedal, which is found to the left of the floor pedals
● Electric or push button, which are found amongst the other console controls
Because most modern braking systems have fail-safe measures and warning systems, such as on-dash brake-warning lights and low-fluid sensors, the emergency brake is most often used as a parking brake device. But the e-brake is called an emergency brake for a reason -- using it can save your life.
The best maintenance for the parking brake is to engage it once a week, to prevent corrosion build-up and sticking. Apply a little lube on the cables and drum/disk contact points at oil changes, and as part of fall/spring maintenance. In most cases, this is all that’s needed to keep the parking brake system working properly.
Common Automotive/Light Truck Brake Systems
A hydraulic brake system is composed of a master cylinder that is fed by a reservoir of hydraulic braking fluid. This is connected by an assortment of metal tubing and rubber fittings which are attached to the cylinders of the wheels. The wheels contain two pistons located on the disk pads or drum shoes. Pushing the brake pedal releases pressurized fluid into the cylinders, which pushes the pistons apart and forces the brake pads or shoes tightly onto the stopping surfaces of the disk or drum, thus causing the wheel to stop moving.
A brake system rising in popularity, electromagnetic brakes use an electric motor that is included in the automobile or light truck which helps the vehicle come to a stop. These types of brakes are in most hybrid vehicles and use an electric motor to charge the batteries and regenerative brakes. On occasion, some buses will use a secondary retarder brake which uses an internal short circuit and a generator.
Pumping brakes are used when a pump is included in part of the vehicle. These types of brakes use an internal combustion piston motor to shut off the fuel supply, in turn causing internal pumping losses to the engine, which causes braking.
Servo brakes augment the amount of pressure the driver applies to the brake pedal. These brakes use a vacuum in the inlet manifold to generate extra pressure needed to create braking. Additionally, these braking systems are only effective while the engine is still running. In some vehicles we may find that there are more than one of these braking systems included. These can be used in unison to create a more reliable and stronger system.
Medium/Heavy Truck (CMV) Air Brake Systems
Air brakes are likely the most important component part of modern commercial motor vehicles. The air brake systems on Commercial Motor Vehicles (CMV) are frictional systems but use compressed air to push the brake pad/shoe onto the disc/drum rather than brake fluid.
As long as the driver, who may be pulling 80,000 pounds of weight, knows how to use them, and the company who owns and services the trucks knows what to look for to adequately maintain the air brake system, they’re as safe and reliable as any other system.
Why air? Several reasons:
● The supply of air in a large diesel truck is unlimited, so the brake system can never run out of its operating fluid, as hydraulic brakes can. Minor leaks do not result in brake failures.
● Airline couplings are easier to attach and detach than hydraulic lines; the risk of air getting into hydraulic fluid is eliminated, as is the need to bleed brakes when they are serviced. Air brake circuits on trailers can be easily attached and removed.
● Air not only serves as a fluid for transmission of force, but also stores potential energy as it is compressed, so it can serve to control the force applied; hydraulic fluid is nearly incompressible. Air brake systems include an air tank that stores sufficient energy to stop the vehicle if the compressor fails.
● Air brakes are effective even with considerable leakage, so an air brake system can be designed with sufficient "fail-safe" capacity to stop the vehicle safely even when leaking.
● The compressed air inherent in the system can be used for accessory applications that hydraulics are not appropriate for, such as air horns and seat adjusters.
How Truck Air Brakes Work
Air brakes work using compressed air rather than hydraulic fluid. As with automobiles and light trucks, they can use either drum brakes, disc brakes or a combination of both.
Air is pressurized by an engine-mounted compressor, which pumps it into the air storage tanks, where it stays until it’s needed.
There are multiple air circuits in the system. Air pressure is used to both apply the service brakes and release the parking brake. The parking brake engages by spring force in the parking brake portion of the spring brake chamber when the air pressure in the chamber is released.
This also allows the parking brake to be used as the emergency brake system. If air pressure was to drop too low, the force exerted by the spring in the chamber will be able to overcome the force exerted by the air on the diaphragm and apply the brakes on all wheels.
You might think of air brakes as working similarly to a hydraulic brake circuit. As with hydraulic brakes, when the driver presses the brake pedal, air pressure is applied, like hydraulic pressure in a hydraulic brake circuit to the wheel when applying the brake.
Air Drum Brake Operation
- The engine-mounted air compressor pumps air into the air storage reservoirs beneath the truck.
- When the brake pedal is pushed, the stored air is released to the brake valves, pushes a rod out and moves the slack adjuster.
- The slack adjuster calibrates the brake system and ensures that the internal spring mechanism is fully extended and working appropriately. If the spring were not fully extended, the system would not create the appropriate friction needed to slow the vehicle.
- Air flows through nozzles to the air brake chamber, which causes the spring to move an S-cam, which forces the brake shoe linings out, away from one another, and presses them inside the brake drum.
- This creates the friction and pressure needed to slow the wheel down.
- When the brake pedal is released, the “S-cam” rotates back, the spring pulls the brake shoes away from the brake drum and eliminates the friction and slow-down effect.
Air Disc Brake Operation
- Once again, the engine-mounted air compressor pumps air into the air storage reservoirs beneath the truck.
- When the operator depresses the brake pedal, the air from the storage reservoirs is supplied through brake lines to the brake valves.
- That air is delivered to the brake chambers, which actuates the brake caliper and transfers force to the inner brake pad.
- The caliper slides on the guide pins, as the inner brake pad contacts the brake rotor.
- A bridge moves with the caliper to move the outer pads against the rotor, where they squeeze against the rotor and transfer force to stop the wheel.
- When the brake pedal is released, the return spring forces the caliper bridge back to its rest position. the brake pads separate from the brake disc and eliminates the friction and slow-down effect.
Air Brake Safety
Because they’re more complex, and their day-to-day workloads often far greater than those of their fluid brake counterparts, air brakes require a great deal more attention. Therefore, it is essential that truck drivers and the companies they drive for are consistently and regularly checking the air brake systems for safe operations and any components that may fail.
Roadside and weigh station motor enforcement inspections can take place at any time and put a CMV out of service where it stands if requirements are not met. In fact, many of the larger private and municipal CMV operators require a daily visual inspection of brake components at both the start and the end of the day, ensuring all visible air brake components are up to par.
The Federal Motor Carrier Safety Administration (FMCSA) of the U.S. Department of Transportation (DOT) maintains strict requirements regulating the thickness of brake linings and pads in trucks, as well as the condition of brake actuators, slack adjusters and drums/rotors.
According to the FMCSA, the thickness of the brake lining and pads of steering axle brakes on a truck, truck-tractor or bus “shall not be less than 4.8 mm (3/16 inch) at the shoe center for a shoe with a continuous strip of lining.” The thickness requirements are at least 6.4 mm (1/4 inch) for two pads. If air brakes are used, the minimum thickness is 3.2 mm (1/8 inch).
The requirements for Non-Steering Axle Brakes states that the thickness of the brake lining and pads of non-steering axle brakes on an air braked commercial motor vehicle shall not be less than 6.4 mm (1/4 inch). If disc brakes are used, the minimum thickness is 3.2 mm (1/8 inch).
Operation Air brake
The Commercial Vehicle Safety Alliance (CVSA) is a nonprofit association comprised of local, state, provincial, territorial and federal commercial motor vehicle safety officials and industry representatives.
The CVSA’s Operation Air brake is a comprehensive program dedicated to improving commercial vehicle brake safety throughout Canada, Mexico and the United States. Each year the alliance sponsors two Brake Safety events, Brake Safety Week and an unannounced one-day brake safety enforcement initiative.
During these events, CMV inspectors conduct Level IV brake Inspections on large trucks and buses throughout North America to identify out-of-adjustment brakes and brake-system violations. Since the program’s inception in 1998, more than 3.4 million brakes have been inspected.
When it comes to providing you with expertise, advice and the leading brands in commercial-grade braking products, Imperial Supplies is ready to help. We offer an unparalleled selection of air brake components, including assemblies, valves, DOT brake fittings, brake chambers, glad hand accessories, air treatment equipment, brake lubricants and more.
In addition to industry-leading products, our Dedicated Account Advisors, technical support and ordering technologies are designed specifically to simplify your day. Together, it all ensures you’ve got the right products on hand, in the right quantities, exactly when you need them. With no mistakes.
Give us a call or reach out to chat whenever you’re ready. Once again, we’re ready to prove to you why your Fleet is Our Focus.