How Truck Brakes Work

truck brake sign
Signs like these apply to big trucks; they have a different brake system than a car.
Photo courtesy of Adam Dodd/

"No Engine Brake." Have you ever won­dered what this sign means when you pass it on the highway?

A truck's engine brake is also known as a­ compression brake system. Sometimes it's called an engine retarding device. Retarders are especially helpful for truckers on mountain downgrades.


But how do retarders work? The compression stroke is one part of an engine's four-cycle system. The intake and exhaust valves are closed as the piston moves up, compressing the air being forced into the brake cylinder. At the top of the stroke, fuel enters the very hot air, burning rapidly and pushing against the piston, which is on its way back down. When the piston begins its ascent, the exhaust valve opens to emit the spent gasses.

Suppose that the valve is popped open when the piston is just past the top center, as the compression­ stroke is ending. The compressed air is allowed to escape. Then the intake valve lets in more air, and the piston begins to compress it. At that point, the engine becomes an air compression device, with gravity providing the energy needed to run it [source: Newbie Driver].

As the compressed air escapes, the truck makes a backfiring or popping sound. Although more modern systems make less noise, the perception remains that a truck should rely on the air brake system.

Now, let's roll on to take a closer look at how truck and automobile brakes differ.



How Are Truck Brakes Different From Car Brakes?

­Let's ­begin by considering the ways that truck and car brakes are al­ike. The purpose of bra­kes on all types of vehicles is to stop them. Brakes on both trucks and cars work on the principle of friction. Both kinds of vehicles have brake drums, along with their pads and shoes, connected to the vehicles' wheel axles.

Car brakes rely on the brake fluid that flows through the system to work properly. Thus, car brakes are a hydraulic system, relying on fluid. On the other hand, truck brakes depend on compressed air. (Trains and buses also use this type of brake system.)


A major plus to using air is that it never runs out (as brake fluid can). This means that the air brake system is very reliable -- even if there's a small leak somewhere in the system, it always works.

Most of the newer heavy trucks use a dual air brake system that is not available on automobiles. A single set of brake controls works both of these separate air brake systems. If one system fails, the other will work.

The flaw in the compressed air system of trucks is brake lag. That's the time it takes for air to get through the lines and force the linings to contact the drum. When they push the brake pedal, drivers must get used to the fact that air brakes don't work at once, as they do on a car. Lag time is less than a second, so this is not a major problem.

The truck's air braking system has several tasks. First, it keeps up a steady supply of compressed air. In addition, it must direct that air's flow. Finally, it uses the energy of air pressure and changes it into mechanical force.

One, two, three! A truck's air brake system is really made up of three different brake systems. Let's investigate each of them.



Truck Brake Types

Service brakes are used during normal driving. A seque­nce of events occurs when a driver pushes the brake pedal.

  1. Air moves into a brake chamber through airlines.
  2. The air forces out a pushrod.
  3. The pushrod pushes the slack adjuster.
  4. The camshaft turns.
  5. The turning of the camshaft twists the S-Cam. (You guessed it -- it's called an S-Cam because it's in the shape of an S.)
  6. The brake linings are forced to contact the brake drum.

The driver activates the parking brakes by pulling out one or both of the valves on the dash. (Tractor brakes have a yellow button; trailer brakes have a red button.) The dash button releases the spring inside the brake chamber, thus beginning steps 3-6 listed above.


The emergency brake system uses parts of the other two systems to stop the truck if brake failure takes place [source: Newbie Driver].

Inside the brake chamber is a powerful spring with about 2,500 pounds of pressure behind it. That spring is held back by a steady and constant airflow in the chamber. The emergency brakes deploy automatically if there's not enough air in the system to hold the spring back. If air pressure drops below 60 pounds per square inch, the low-pressure light comes on. A buzzer may also sound.

Not everyone is happy with the way that S-Cam brakes work. A recent innovation is the use of disc brakes. Let's head down the highway to see how disc brakes work on trucks.



Big Disc Brakes for Trucks

Some trucks have disc brakes instead of S-Cam brakes. Air pressure works on the brake chamber and the slack adjuster in the same way that it does in S-Cam brakes. However, a power screw replaces the S-Cam. The power screw is turned by t­he pressure on the brake chamber and the slack adjuster. Then the power screw grasps the disc or rotor between a caliper's brake lining pads.

The many benefits of air disc brakes include the following:


  • Compared to drum brakes, air disc brakes reduce stopping distances by almost 40 percent. Because pressure is continually applied, it's easier for the vehicle to come to a complete stop.
  • Air disc brakes almost completely stop brake fade.
  • Air disc brakes don't increase the wear of brake linings on a trailer or disc pads on a tractor.

Still, despite all of the advantages, change isn't always embraced by the masses. In the United States, no regulation demands the use of air disc brakes, which are viewed as a high-end item because they're about twice the cost of drum brakes. Currently, about 95 percent of the U.S. heavy-duty truck market relies on drum brakes. In Europe, more than 80 percent of commercial trucks already use air disc brakes. The complete shift to air disc brakes may require 10 to 15 years.

They say that a picture is worth a thousand words. Let's cruise to the next page to get a glimpse of a diagram of a truck's braking system.



Truck Brake Diagram

air brake diagram, air-brake diagram
Diagram of air-brake components

Now let's put the parts together to see how truck brakes work as a whole. This diagram provides both a closeup view and an example of where the brakes are located in your vehicle.


Lots More Information

Related ­HowStuffWorks Articles


  • "Air Brakes 101." (10/31/08). (Accessed 11/5/08)
  • Britannica Concise Encyclopedia. "Air Brake." (10/31/08). (Accessed 11/5/08)
  • Bulk Transporter. "Bendix Spicer JV Promotes Air Disc Brakes." (Accessed 11/5/08)
  • California Department of Motor Vehicles. "Commercial Driver License Handbook. Section 5: Air Brakes." (10/31/08). (Accessed 11/5/08)
  • "George Westinghouse, Jr." (11/01/08). (Accessed 11/5/08)
  • Grygier, Paul A. "Study of Heavy Truck Air Disc Brake Effectiveness on the National Advanced Driving Simulator." (11/03/08). pdf file downloaded from - 2007-04-05 (Accessed 11/5/08)
  • National Transportation Safety Board. "Lessons Learned from a Fatal Crash." (10/31/08). (Accessed 11/5/08)
  • New York State Commercial Driver's Manual. (10/31/08). (Accessed 11/5/08)
  • Thomas, Ed. "Retarders." (11/1/08). (Accessed 11/5/08)
  • U.S. Department of Transportation. "U.S. Transportation Secretary Mineta Announces FMCSA Rule Permitting Performance Brake Testing Technology." (10/31/08). (Accessed 11/5/08)