The air brakes test is one of the most challenging CDL endorsement exams. Unlike hydraulic brakes in passenger cars, air brake systems are complex and require specific knowledge to operate safely. If you're driving any commercial vehicle equipped with air brakes, you need to pass this test. Here's your complete study guide.
Why Air Brakes Require a Separate Test
Air brakes work fundamentally differently from hydraulic brakes. They use compressed air instead of fluid to apply braking force. Because of this, they have unique characteristics like brake lag, air pressure requirements, and the potential for brake fade that every commercial driver must understand.
Key Components You Must Know
Air Compressor
Pumps air into the storage tanks. It's driven by the engine through gears or a belt. The governor controls when the compressor pumps air — it cuts in at around 100 psi and cuts out at around 125 psi.
Air Storage Tanks
Hold compressed air for braking. Tanks must be drained regularly to remove moisture and oil buildup. Accumulated water can freeze in cold weather and cause brake failure.
Brake Pedal (Foot Valve/Treadle Valve)
Controls air pressure applied to the brakes. Pushing harder applies more air pressure and more braking force.
Foundation Brakes
The actual braking mechanism at each wheel. Most common type is the S-cam drum brake. The S-cam turns, pushing the brake shoes against the drum.
Slack Adjusters
Connect the push rods to the brake S-cams. They must be properly adjusted — if push rod travel exceeds the limit, brakes won't work effectively. Many modern trucks have automatic slack adjusters.
Spring Brakes (Parking Brakes)
These are critical to understand. Spring brakes use powerful springs to apply the brakes when air pressure is released. They work opposite to service brakes — air pressure holds them OFF, and loss of air lets them apply.
Critical Numbers to Memorize
- Cut-in pressure: ~100 psi (compressor starts pumping)
- Cut-out pressure: ~125 psi (compressor stops)
- Low air warning: Must activate before 60 psi
- Spring brakes apply: 20-45 psi range
- Air pressure buildup: 85-100 psi within 45 seconds
- Maximum air loss rate (single): No more than 3 psi in 1 minute
- Maximum air loss rate (combination): No more than 4 psi in 1 minute
- Brake lag distance at 55 mph: ~32 feet
- Push rod travel limit (Type 30): 2 inches
The Air Brake Test (Applied Leakage Test)
During the CDL skills test, you'll need to perform an air brake check:
- Build air pressure to governor cut-out (around 125 psi)
- Turn off the engine
- Release the parking brake (push in the yellow valve)
- Apply the foot brake fully
- Watch the air pressure gauge — it should not drop more than 3 psi in one minute (4 psi for combination vehicles)
- Check that the low air warning activates before pressure drops to 60 psi
- Continue pumping the brake — spring brakes should pop out between 20-45 psi
Brake Lag & Stopping Distance
Unlike hydraulic brakes that respond instantly, air brakes have a delay called brake lag — the time it takes for air to travel through the lines and apply the brakes. This adds about half a second to your reaction time.
At 55 mph, total stopping distance for a loaded truck with air brakes is approximately 390 feet — that's longer than a football field.
- Perception distance: 60 feet
- Reaction distance: 60 feet
- Brake lag distance: 32 feet
- Braking distance: 238 feet
- Total: 390 feet
Common Test Questions
- What happens when you lose air pressure? (Spring brakes apply automatically)
- Why drain air tanks? (Remove moisture that can freeze or corrode)
- What is brake fade? (Brakes overheat and lose effectiveness on long downgrades)
- How to avoid brake fade? (Use proper gear for downgrades, use the snub braking technique)
- What does the governor do? (Controls when the air compressor pumps air)
- Maximum leakage rate? (3 psi/min single, 4 psi/min combination)
Practice Air Brakes Questions
CDL Test Prep has dedicated air brakes practice tests with detailed explanations for every answer.
Download Free on Google Play