All the air brake systems are using the air pressure for the brakes every time the pedal is stepped on. The air is being stored on several reservoirs or pressure tanks in the truck. The stored air are pressurized using the air compressor that is located in the engine of the truck. The air pressure is being regulated by the air governor in the compressor and most of this works on the level of 120 psi pressure. The governor manipulates when air compressor pump an air in the pressure tanks. If the air reservoir pressure exceeds the cut out level, the governor halt the compressor in pumping air.
Some systems possess air dryer that dries all moisture so that the water condensation would not build in the pressure tank causing brake failures. This will also avoid freezing during winter season. The air dryer has cartridge that needs to be replaced once a year. The air is being pressured by the compressor that passes in the air dryer down to the tanks. And from the air tanks, it goes to different type of systems. The first one is the emergency or the supply system, which discharges parking brakes and keep the reserve air to halt whenever there is a leak.
The manner that the system of parking brakes works on the truck is through the use of springs in air chambers in rear axles of the truck which puts the parking brakes. If the parking brakes knob is pushed, it supplies air in the air chamber which pushes against its springs to discharge the parking brakes. When the air pressure in air system goes down below 60 psi, the parking brake knob would pop out and will set parking brakes. This is a kind of safe feature to make the air truck stop because when the pressures fall down, the service brake might not work.
When driving in a semi-tractor that has trailer, the trailer knobs in the brakes works on the same manner. But if an air leak exist in the trailer, the protection valve in the tractor will create the trailer brake as protection to the air supply so that the truck would still stop. The other kind of system is called the service brake. It is a kind of system that stops you whenever pushing on the pedal break. When the brake pedal is pushed on, it goes to the pedal valve, which in return control the flow of the air in brake chambers.
Brake chambers also push the slack adjusters, a component that maintains the brake in adjustment. There are also the so-called disc air brakes which are unpopular. Most of brake shoes have the indicator that is built at the end of pads which needs to be replaced in a regular period. When changing the brake shoes, the hardware kits is also needed which includes the pins, busing, and springs for the brakes. The drums must also be replaced as the brake shoes are replaced. This is important because the drums might wear ridge on it that can trigger heat cracks.
If you would like to take a practice test on air brakes please visit: CDL Learning Center