CDL Air Brakes: Air Compressor, Air Compressor Governor, and Air Storage Tanks
Air Brakes Basics: The Three Systems, the Compressor, the Governor, and Air Tanks
If you plan to drive a commercial vehicle equipped with air brakes, you need a solid understanding of how those brakes work. This article introduces the air brake section of the CDL manual, explains the three separate braking systems that make up an air brake setup, and begins looking at the individual parts — starting with the components responsible for creating, regulating, and storing compressed air.
Who Needs to Study Air Brakes
Any driver who wants to operate a commercial motor vehicle equipped with air brakes needs to study this material. If you'll also be pulling a trailer that has air brakes, you'll additionally need to review the section on combination vehicles.
As the name suggests, air brakes rely on compressed air to make the brakes work. They're widely considered a good and safe way to stop large, heavy vehicles — but that safety depends on two conditions: the brakes must be well maintained, and they must be used properly.
Three Systems in One
What looks like a single braking setup is actually three distinct systems working together:
- The service brake system: This is what applies and releases the brakes when you press the brake pedal during normal driving.
- The parking brake system: This applies and releases the parking brakes whenever you use the parking brake control.
- The emergency brake system: This system borrows parts from both the service and parking brake systems to bring the vehicle to a stop if the brake system fails.
The Air Compressor
Everything in an air brake system depends on a steady supply of compressed air, and the air compressor is what provides it. Its job is to pump air into the air storage tanks, which are also called reservoirs.
The compressor is driven by the engine, connected either through gears or through a v-belt. Depending on the vehicle, the compressor may be air cooled, or it may be cooled by the engine's own cooling system. Lubrication varies as well — some compressors have their own dedicated oil supply, while others are lubricated by the engine's oil. If your compressor has its own oil supply, make sure to check the oil level before you drive.
The Air Compressor Governor
The compressor doesn't run at full output all the time. Instead, a component called the governor decides when it should be pumping air into the tanks and when it should stop.
The governor works using two pressure points. When the pressure in the air tanks climbs to the "cut-out" level — around 125 pounds per square inch (psi) — the governor tells the compressor to stop pumping. Later, as air gets used and tank pressure falls to the "cut-in" level, which is around 100 psi, the governor allows the compressor to start pumping again. This cycle keeps the air supply within a safe operating range without the compressor running constantly.
Air Storage Tanks
Compressed air is held in air storage tanks. The number and size of these tanks differ from vehicle to vehicle. One important safety feature of the tanks is their capacity: they're designed to hold enough air that the brakes can be applied several times even if the compressor stops working. That reserve gives a driver time to react and bring the vehicle to a safe stop if something goes wrong with the air supply.
Together, the compressor, governor, and storage tanks form the foundation of the air brake system — the part responsible for making, regulating, and holding the compressed air that every other component depends on.