CDL Air Brakes Sample Questions and Explanations

Wearing gloves, pull hard on each slack adjuster you can reach. If it moves more than about 1 inch where the push rod attaches, it probably needs adjustment. Brakes with too much slack can make the vehicle very hard to stop.

Wrong choices

• More than about 1/4 inch: A small amount of movement is normal. The manual uses about 1 inch as the limit, not 1/4 inch.
• More than about 3 inches: Waiting until there are 3 inches of movement is far too late. The brake would already be badly out of adjustment.

Check the build-up rate with the engine at operating rpm. In a dual air system, pressure should build from 85 to 100 psi within 45 seconds. (If the vehicle has air tanks larger than the minimum size, build-up can take longer and still be safe. Check the manufacturer's specifications.)

Wrong choices

• Within 3 minutes: 3 minutes is the typical figure for older single air systems building from 50 to 90 psi at idle, not for dual systems.
• Within 10 minutes: 10 minutes is much too slow. Pressure could drop too low while you drive.

An air brake system combines three braking systems: the service brake (used in normal driving when you press the pedal), the parking brake (used when you park), and the emergency brake (uses parts of the service and parking brakes to stop the vehicle if the service brake system fails).

Wrong choices

• Service brakes, engine brakes, and exhaust brakes: Engine and exhaust brakes are speed retarders, not part of the three air brake systems.
• Front brakes, rear brakes, and trailer brakes: These are places where brakes are, not the three braking systems.

The air compressor governor controls when the compressor pumps air into the tanks. When tank pressure is high enough, it stops the pumping; when pressure falls, it starts the pumping again.

Wrong choices

• The safety relief valve: The safety relief valve only opens to let air out if pressure gets too high.
• The foot valve: The foot valve (brake pedal) sends air to the brakes; it does not control the compressor.

Controlled braking means applying the brakes as hard as you can without locking the wheels. Keep steering movements very small while doing it.

Wrong choices

• Braking fully, releasing when wheels lock, and braking again: Braking fully and releasing when the wheels lock is stab braking, a different method.
• Pumping the brakes lightly and quickly: Light, quick pumping is not how the manual describes controlled braking.

Stab braking:

1. Apply the brakes all the way.
2. Release them when the wheels lock up.
3. As soon as the wheels start rolling, apply the brakes fully again.

Wrong choices

• Brake as hard as you can without ever locking the wheels: Braking hard without locking the wheels is controlled braking.
• Pull the parking brake knob and steer straight: The parking brakes are not used to stop a moving vehicle in an emergency.

Most heavy vehicles use dual air brake systems for safety. A dual system has two separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, and so on.

Wrong choices

• One air system with two compressors: A dual system has two separate air systems, not just two compressors.
• An air brake system with both spring brakes and service brakes: All air brake vehicles have both; that is not what "dual" means.

Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi pressure in both the primary and secondary systems. Watch the primary and secondary air pressure gauges (or needles, if the system has two needles in one gauge).

Wrong choices

• At least 60 psi in either system: 60 psi is the low air warning level; it is not enough to start driving.
• At least 100 psi in the primary system only: Both systems must reach at least 100 psi.