To bypass Intro, click on 1:22 ... Section 5 Air Brakes CDL Manual Read Along (Entire Text) Studying on the Move Transcript with Larger Font: Section 5 Air Brakes This section covers: • Airbrake System Parts, • Dual Airbrake, Systems, • Inspecting Airbrakes, and • Using Airbrakes

This section tells you about airbrakes. If you want to drive a truck or bus with airbrakes, or pull a trailer with airbrakes, you need to read this section. If you want to pull a trailer with airbrakes, you also need to read Section 6, Combination Vehicles.

Airbrakes use compressed air to make the brakes work. Airbrakes are a good and safe way of stopping large and heavy vehicles, but the brakes must be well maintained and used properly.

Airbrakes are really three different braking systems: service brake, parking brake, and emergency brake.

• The service brake system applies and releases the brakes when you use the brake pedal during normal driving.
• The parking brake system applies and releases the parking brakes when you use the parking brake control.
• The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.

The parts of these systems are discussed in greater detail below.

5.1 – The Parts of an Airbrake System
There are many parts to an airbrake system. You should know about the parts discussed here.

5.1.1 – Air Compressor
The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a vbelt. The compressor may be air cooled or may be cooled by the engine cooling system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.

5.1.2 – Air Compressor Governor
The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the “cutout” level (around 125 pounds persquareinch or “psi”), the governor stops the compressor from pumping air. When the tank pressure falls to the “cutin” pressure (around 100 psi), the governor allows the compressor to start pumping again.

5.1.3 – Air Storage Tanks
Air storage tanks are used to hold compressed air. The number and size of air tanks varies among vehicles. The tanks will hold enough air to allow the brakes to be used several times, even if the compressor stops working.

5.1.4 – Air Tank Drains
Compressed air usually has some water and some compressor oil in it, which is bad for the airbrake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure that you drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom. There are two types:

• Manually operated by turning a quarter turn or by pulling a cable. You must drain the tanks yourself at the end of each day of driving. See Figure 5.1.
• Automatic—the water and oil are automatically expelled. These tanks may be equipped for manual draining as well.

Automatic air tanks are available with electric heating devices. These help prevent freezing of the automatic drain in cold weather.

5.1.5 – Alcohol Evaporator
Some airbrake systems have an alcohol evaporator to put alcohol into the air system. This helps to reduce the risk of ice in airbrake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.

Check the alcohol container and fill up as necessary, every day during cold weather. Daily air tank drainage is still needed to get rid of water and oil. (Unless the system has automatic drain valves.)

5.1.6 – Safety Valve
A safety relief valve is installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve releases air, something is wrong. Have the fault fixed by a mechanic.