Service brakes are used during normal driving. A sequence of events occurs when a driver pushes the brake pedal.
- Air moves into a brake chamber through airlines.
- The air forces out a pushrod.
- The pushrod pushes the slack adjuster.
- The camshaft turns.
- 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.)
- 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.