Air Brakes Test Study Guide for the CDL

Page 1

General Information

Knowledge of the air brake system is essential for anyone desiring to obtain a Commercial Driver’s License that allows operation of a vehicle with air brakes. This study guide outlines the information you need to know to safely operate such a vehicle.

Parts of an Air Brake System

The air brakes system has three parts: the service, parking, and emergency brake systems. Within these systems, there are many main parts with which one needs to be familiar.

  • The air compressor pumps air into the storage tanks.

  • The air compressor governor controls when the compressor will pump air into the storage tanks.

  • The air storage tanks hold the compressed air used in the braking system.

  • The air tank drains drain built up water or oil in the bottom of the tanks.

  • The alcohol evaporator puts alcohol into the air system to reduce the risk of ice.

  • The safety valve automatically opens at a set PSI (pounds per square inch), usually 150, to prevent too much pressure in the air brake system.

  • The brake pedal engages the air brakes when pressed.

  • Foundation brakes are the brakes used at each wheel. They are most commonly S-cam brakes but can also be wedge brakes or disc brakes.

  • Supply pressure gauges are connected to the air tanks and indicate the pressure in each tank.

  • The application pressure gauge displays how much pressure is being applied to the brakes. (Not all trucks are equipped with application pressure gauges.)

  • The low air pressure warning alerts when the air brake pressure falls below a safe amount (usually 60 PSI).

  • The stop light switch turns on the brake lights when applying the brakes, alerting anyone behind that the vehicle is slowing.

  • The front brake limiting valve is only on vehicles made before 1975. When set to “slippery,” it cuts the standard air pressure to the front brakes in half.

  • Spring brakes are used for the emergency and parking brake systems. Powerful springs are held back by air pressure, and when that air pressure is released, the springs allow for braking. Spring brakes are not installed on steering axles. Steering axles are not part of the parking or emergency braking systems. Steering axles are part of the service brake system only.

  • The parking brake control is usually a yellow, diamond-shaped, push-pull control knob. On older vehicles, the parking brake may be controlled by a lever.

  • The trailer protection valve is usually a red, octagon-shaped, push-pull control knob.

Here are some diagrams to help you understand the system and how it functions:

1 Air Brake System FIXED.jpg

Retrieved from:

2 Drum Brake FIXED.jpg

Retrieved from:

Dual Air Brake Systems

Dual air brake systems are used by most new vehicles. In a dual system, there are two separate air brake systems, each with its own lines, hoses, air tanks, etc., but both run by the same set of brake controls. Typically, one system (the primary system) controls the regular brakes on the rear axle(s), and the other system (the secondary system) controls the brakes on the front axle (and maybe one rear axle).

Inspecting the Air Brakes

It is very important to keep the brake system in good repair. Key to a properly functioning truck is constant inspection, and general awareness of the equipment. Daily inspection of the air brake system include:

  • While pre-tripping under the hood, check the condition of all equipment. Most trucks have gear-driven air compressors—but if your air compressor has a belt, check the belt’s condition. Grasp and lightly shake various components under the hood, to test for properly secured equipment.

Note: Never inspect under the hood while the engine is running.

  • While performing your daily walk around, look for signs of wear, broken equipment, or loose/dangling hoses. Shiny bits of metal and/or streaks of rust are clear signs of rubbing or loose equipment.

  • Underneath the truck and/or trailer, the brake drums should not be cracked more than 1/2 of its width. Linings (friction material) should be clean. There should be no oil or grease on the linings. Check for missing parts. Hoses should be without cuts or tears. Shiny patches on hoses could mean rubbing surfaces, and should be investigated. Ty-wraps and duct tape are clear indications that something is wrong.

  • While driving, if the truck “pulls” to one side while braking, it might be a sign that your slack adjusters (or other brake parts) are out of adjustment. Particularly examine if the truck pulls one direction under light braking and pulls the opposite direction under hard braking. This is a clear indication that something is wrong.

Hint: If you are driving in wet, rainy, snowy, or icy conditions, lightly press the brakes as you first start out, for 10–15 seconds. Pulling against lightly-applied brakes will heat the drums and help dry out the brake components. This will provide better braking.

  • While you are fueling tanks, it is an easy thing to give the air-tank drain-cables a quick pull. If it is a manual drain valve, it should be drained of water and contaminants at the end of every day’s driving.

Operating the Air Brakes

In a normal stop, to operate air brakes, you simply push the brake pedal down using controlled pressure. To perform an emergency stop, there are two methods you can use.

  • The first is controlled braking, which means applying the brakes as hard as possible without locking the wheels.

  • The second, stab braking, means pressing on the brake pedal until the brakes lock up, releasing the brakes, and then as soon as the wheels begin rolling again, pressing the pedal back down as hard as you can.


All Study Guides for the CDL are now available as downloadable PDFs