- A brake actuator is an air-operated relay system that controls the rear brakes of a truck or tractor-trailer. Brake actuators are an integral part of a larger air-brake system on large vehicles. The purpose of a brake actuator is to provide air to a truck's rear brakes quickly enough for the rear brakes to engage at the same time as the front brakes. In smaller vehicles, the brake pedal releases air that modulates the brake pads on all four wheels. However, in larger trucks it would take too long for the air to travel from the brake pedal to the rear brakes, causing the rear brakes to engage after the front brakes. To correct for this lag, a brake actuator system stores a supply of air near the truck's rear brakes, and this air supply is released when the brake pedal is depressed, causing the rear brakes to engage more quickly.
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Diagram of a Brake ActuatorBrake actuator systems contain an air supply and a quick-release system that delivers this air to the brake chambers when the brake pedal is applied. A wide pipe connects the air supply to the relay valve, and the air supply also contains an electronic sensor that is attached to the brake pedal by a long wire. The electronic sensor knows when the brake pedal is applied and causes the air to be released through the pipe and into the relay valve. The quick-release system in the relay valve pushes this air into the brake chambers at very high pressure. This pressurized air acts on the brake pedals, causing them to engage with the rear wheel's rotors. - Trucks with brake actuator systems contain an additional gauge on their dashboards. This gauge, known as a brake application gauge, indicates the air pressure in the rear brake chambers when the brakes are applied. The brake application gauge alerts the driver if the air pressure in the brake chambers is too low, which would slow the braking action of the rear brakes and make it more difficult for the truck to stop. Many brake actuator systems contain an alarm of some sort that will go off if the air supply or the relay valve is leaking air. In most cases, the alarm will be triggered if the air pressure in the brake actuator system falls below 60 psi.
