I have tried to cover all the details, but because brake systems vary in design, you should consult your vehicle’s service information or speak to a professional technician for specific procedures and specifications. Your local auto parts store can be a big help too.
Automotive Brake Systems
Believe it or not, the first vehicles had no brakes. Drum brakes were the first brake system used on early vehicles, but they did not use brake shoes like we know today.
History
The first designs used a flexible friction band that clamped around the outside of the drum that the wheel bolted to, like a band in an automatic transmission.
Most vehicles had them only on the rear axle. When the brake pedal was applied, a long metal rod from the brake pedal moved a lever that squeezed the band around the spinning drum, hopefully bringing it to a halt. No hydraulics were used at that time and this design was also subject to water contamination and rust since it was exposed to the elements. Needless to say, the system required a lot of pedal force to stop. You might think vehicles didn’t go fast enough to require much force, but actually vehicles in that era could achieve highway speeds and were extremely heavy, so accidents were pretty common due to inadequate brakes.
The external band (arrow) that applied around the drum was very similar to those used inside of an automatic transmission but were exposed to the elements. As a result, they were greatly affected by rain and dirt contamination. Rust was also a common problem with this design.
Eventually, internal brake shoes replaced the external band on drum brake systems and they were added to all four wheels, but hydraulics were still not used. The systems remained mechanical. This made it difficult for brake engineers because the front wheels must turn, and the mechanical linkages to the brakes must accommodate this. Enter hydraulics.
This 1919 Buick has a basic rear brake–only system that squeezes friction bands around spinning drums with mechanical rods to the brake pedal. Highway speeds were possible with this vehicle, which made braking an interesting experience.
Mechanically applied drum brake systems used long metal rods between a lever at the brake pedal and levers at the rear axle (arrows). Because the axle moves up and down when the suspension moves, brake pulsations could occur when stopping.
Hydraulics provide the same function as mechanical rods but are flexible and can route around obstructions and move with the suspension and steering systems. The term “hydraulic advantage” is used to explain how different-sized pistons can increase force inside of a brake system. If an input piston attached to the brake pedal is smaller than the output piston that applies the brake shoe or pad, the force is increased.
When brake engineers added hydraulics to apply the shoes inside the drums, brake systems became extremely effective and reliable. Hydraulic fluid within a closed system acts like a solid metal rod, but it can be routed around components and pass through flexible hoses so that the front wheels can turn and the vehicle suspension can move freely. Hydraulic principles also allow for different piston diameters to be used between the master cylinder (input at the driver’s foot) and the wheel cylinder (output to the brake shoes) so that pedal force and travel can be varied to best suit the driver and vehicle type.
Some of these early designs use a wheel cylinder with a single piston pushing on only one shoe. When the shoe touches the spinning drum, rotational forces cause energy to transfer to the other shoe and wedge it into the drum to stop the vehicle. This is known as self-energizing, or servo action.
The typical drum brake system has remained virtually unchanged for many decades. A hydraulic wheel cylinder expands when the brakes are applied, which presses the brake shoes into the inside of the spinning brake drum to stop the vehicle.
Brake Adjustments
Today’s rear drum brake systems look very similar to those on a 1957 Chevy, but early vehicles required routine brake adjustments to maintain pedal height as the brake shoes wore away. As a child, I recall seeing signs at nearly every gas station that read, “Brake adjustments while you wait: $3.” ■
In those early days, drum brakes were used on all four wheels and they really haven’t changed much since then. Early drum brake systems did not automatically adjust, but modern vehicles with drum brakes have automatic adjuster systems that take care of this for you provided everything is assembled properly and not rusted or damaged.
Four-wheel drum power brakes with automatic adjusting capability became commonplace in the late 1960s and early 1970s. A vacuum-controlled booster was added between the brake pedal and the master cylinder that greatly increased the force, allowing virtually anyone to stop a heavy vehicle with minimal pedal effort. This system was a great improvement, but drum brakes are susceptible to water and use many moving parts that can develop problems. If water got in one front brake during a rainstorm or one side was not properly adjusting, the vehicle pulled to one side when stopping. Drum brakes also tend to retain heat and generate a lot of wear material that can get between the friction surfaces, reducing efficiency.
Then along came disc brakes. They were first used in aviation and have many advantages over drum brakes. They can shed dust and squeegee water off as soon as they are applied. They can dissipate heat well and automatically adjust to wear hydraulically, requiring no mechanical methods. They were first used on the front, and drums remained on the rear, but most of today’s vehicles use disc brakes on all four wheels. They are extremely reliable and easier to service.
Some early drum brake systems used a wheel cylinder with only one piston, such as on this 1952 Ford F2 pickup truck. The piston (arrow) pushes the front shoe into the spinning drum. The spinning drum causes force to be applied to the other shoe, which wedges it into the drum and stops the vehicle. This concept is known as self-energizing or servo action. This truck has four-wheel drum brakes with no assist.
A conventional disc brake system uses a brake caliper over a spinning rotor. When the brakes are applied, piston(s) in the caliper (A) press brake pads (B) into the rotor (C) to stop the vehicle. Disc brakes are widely used on today’s cars and light trucks because they are superior to drum brakes in almost every way.
Electronics and Brake Systems
Antilock brakes were first used on trains and airplanes and became mainstream in the United States in the mid-1980s on the rear of many pickup trucks and vans. ABS provided a solution to the problem of rear wheel lockup when the vehicle was unloaded, which could lead to an accident, especially on a curve in the rain. As systems were improved and better sensors and data processing came about, ABS was added to all four wheels. The main goal of ABS is to allow a driver to steer around obstructions during a panic stop. It does this by keeping the wheels from totally locking, which would eliminate the ability to steer.
An antilock braking system (ABS) uses a computer