The braking system of a truck operates pneumatically. Both the motor vehicle (tractor) and the trailer (semi-trailer or trailer) are braked and held stationary using air. Air pressure is generated in the air supply section and is metered via the foot brake pedal to the air-consuming section. The air-consuming section contains the brake cylinders, which press the brake pads against the brake disc or drum via a mechanism. When stationary, the parking brake is activated. The air is expelled from the system, and the powerful springs in the spring brake cylinders engage the rear brakes. To release the brakes again, air pressure is sent from the parking brake valve to the spring brake cylinders to compress the spring again. The brakes are then released.
The pages covering the basic principles of air brakes, air supply, service, and auxiliary/parking brakes, and trailer brakes are described according to the conventional braking system. These still contain many components such as an ALR, which we no longer see as a separate component under the vehicle with the introduction of EBS.
Modern motor vehicles and trailers/semi-trailers use the electronic braking system (EBS), where the brake pedal position is measured by sensors and sent to the EBS control unit. Via CAN bus, the desired braking deceleration is sent to the modulators. The front axle, rear axle, and trailer modulators send the brake pressure, depending on the load, to the brake cylinders. The EBS system offers many advantages over the conventional braking system: due to electrical control, a significant brake deceleration is possible with little pedal effort, and better harmonization between motor vehicle and trailer can be achieved.