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Quizzes > Science

What air brake equipped vehicles have: CDL air brakes quiz

Quick, free CDL air brakes quiz to test your knowledge. Instant results.

Editorial: Review CompletedCreated By: Philip LelliottUpdated Aug 24, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration showing brake drums pressure gauges dual air brake setup icons over a golden yellow background

This quiz helps you confirm what air brake equipped vehicles have and how key parts work. Answer quick questions on drums, reservoirs, gauges, and dual systems, then see instant results to spot gaps before your CDL test. For deeper practice, try the spring brakes quiz and trailer tanks air supply.

Which component compresses air for the brake system?
Brake chamber
Air dryer
Air compressor
Pressure gauge
The air compressor is driven by the engine and supplies compressed air to the braking system, creating the pressure needed to apply the brakes. Brake chambers convert that air into mechanical force, while the air dryer removes moisture and the gauge simply measures pressure. Without the compressor, the system cannot build air pressure.
What stores compressed air in an air brake system?
Check valve
S-cam
Fuel tank
Air reservoir
An air reservoir (tank) stores compressed air until it is needed by the braking system. S-cams and check valves are part of the brake application and control functions, while fuel tanks hold diesel, not air. Proper reservoir capacity is crucial for reliable braking.
Which device removes moisture and oil from compressed air?
Drain cock
Check valve
Air dryer
Air governor
The air dryer is designed to eliminate moisture and oil from the compressed air before it enters the reservoir, preventing corrosion and freezing. The governor controls compressor cut-in and cut-out, while check valves and drain cocks serve different functions. Moisture in the system can lead to ice blockages.
What indicates system air pressure to the driver?
Tachometer
Brake light
Speedometer
Pressure gauge
A pressure gauge displays the current air pressure in the braking system to the driver. Tachometers measure engine speed, speedometers track vehicle speed, and brake lights signal braking to others. Monitoring system pressure is vital for safe operation.
Which device prevents air from flowing back into the compressor?
Foot valve
Check valve
Brake chamber
Relay valve
Check valves allow air to flow in one direction - from the compressor to the tanks - and prevent backflow into the compressor. Relay valves accelerate brake application, foot valves are brake pedals, and brake chambers apply force. Backflow could damage the compressor.
What is the normal pressure range maintained in most air brake systems?
50-70 psi
100-120 psi
160-180 psi
30-50 psi
Most heavy vehicle air brake systems maintain between 100 and 120 psi for safe braking performance. Pressures much below 85 psi can trigger warnings, and systems usually cut out governor at around 120 - 135 psi. Lower ranges are insufficient, while extremely high pressures risk component failure.
Which part converts compressed air into mechanical force to apply brakes?
Slack adjuster
Brake chamber
Pushrod
Brake drum
Brake chambers use compressed air to push a diaphragm and pushrod, converting air pressure into mechanical force that applies the brakes. Pushrods and slack adjusters transmit this force, while drums receive the braking torque but do not produce it. Without brake chambers, air brakes would not function.
Which brake design is most common on heavy-duty air brake systems?
Wedge brakes
Vacuum brakes
S-cam brakes
Disc brakes
S-cam brakes use an S-shaped cam to push brake shoes outward, and are the most common design on heavy trucks and buses. Disc brakes are less common on older heavy vehicles, wedge brakes are more specialized, and vacuum brakes are outdated. S-cam's simplicity and reliability make it preferred.
In a dual air system, how many independent air circuits are there?
Two
One
Three
Four
A dual air system has two completely separate air circuits so that if one fails, the other can still operate brakes on one axle or set of wheels. Single systems have one circuit, and three or four circuits are not standard. This redundancy enhances safety.
What safety device warns the driver of low air pressure?
Spring brake
Air compressor governor
Brake light
Low-pressure warning buzzer
A low-pressure warning buzzer or light alerts the driver if system pressure falls below about 60 psi, indicating a potential loss of braking ability. The governor regulates pressure, spring brakes apply if pressure is lost, and brake lights signal stopping to other drivers. Early warning is critical.
What component controls the cut-in and cut-out of compressor operation?
Spring brake
Check valve
Slack adjuster
Air governor
The air governor senses system pressure and signals the compressor to start (cut-in) and stop (cut-out) charging the air tanks. Slack adjusters adjust brakes, spring brakes apply during emergencies, and check valves prevent backflow. Proper governor settings maintain the correct pressure range.
What is the primary purpose of a relay valve in an air brake system?
Remove moisture from air
Speed up brake application by relaying the driver's signal
Control governor cut-in
Measure reservoir pressure
Relay valves use air pressure from the reservoir to apply the brakes faster in response to a driver's foot valve input, reducing lag in large vehicles. They do not remove moisture, measure pressure, or control the governor. Quick response is essential for even braking.
Which brake component uses heavy springs to apply the brakes if air pressure is lost?
Rotary valve
Tandem air chamber
Spring brake chamber
Foot valve
Spring brake chambers contain powerful springs that automatically apply the brakes if air pressure collapses, acting as an emergency and parking brake. Tandem chambers combine service and parking, rotary valves distribute air, and foot valves are the accelerator for service brakes. This failsafe prevents runaway.
During an applied leakage test, what is the maximum allowable air loss in one minute for a single system at 100 psi?
8 psi
5 psi
3 psi
10 psi
When the brakes are fully applied in a single system at 100 psi, a leak rate of no more than 3 psi per minute is allowed by regulation. Exceeding this indicates faulty seals or connections. This ensures reliability under sustained braking.
What is the function of slack adjusters in S-cam brakes?
Maintain correct brake shoe clearance
Control compressor output
Regulate reservoir pressure
Drain condensate
Slack adjusters link the S-cam to the brake chamber pushrod and allow adjustment to maintain proper clearance between brake shoes and drums. Without correct adjustment, brakes can drag or have excessive pedal travel. They do not handle air or moisture.
How often should air tanks be drained as part of a daily pre-trip inspection?
Daily or as needed to remove moisture
Weekly
Every 1000 miles
Monthly
Draining air tanks daily removes accumulated moisture that can freeze or cause corrosion. Waiting longer can compromise system integrity. It's a key pre-trip step in official inspection checklists.
What can occur if the air dryer fails in freezing weather?
Moisture can freeze and block air lines
System pressure dramatically increases
Compressor governor shuts down
Brakes apply automatically
If the dryer does not remove moisture, water can freeze in lines or valves, blocking airflow. This can cause brake failure or unpredictable operation. Governors and compressors are unaffected directly by moisture.
Which test checks for system leaks without applying service brakes?
Maximum leakage test
Applied leakage test
Governor cut-out test
Static leakage test
A static leakage test measures how much air pressure is lost with the system at operating pressure and brakes released. It detects leaks in lines, tanks, and compressor. Applied leakage tests use full brake application instead.
If one circuit fails in a dual-circuit air brake system, can the vehicle still stop safely?
Only if lightly loaded
Only at slow speeds
Yes, the other circuit will provide braking
No, both circuits must function
Dual-circuit systems are designed so that if one circuit fails, the other can still operate brakes on one axle or set of wheels, allowing a safe stop. It's a legal requirement for redundancy. Relying on a single circuit is dangerous.
How do you test the low-pressure warning signal on an air brake system?
Apply parking brake only
Drain air until signal activates around 60 psi
Run compressor at high RPM
Open all valves simultaneously
To test the low-pressure warning, you slowly drain air using the drain cock or pedal until the warning light or buzzer activates, typically between 55 and 75 psi. This confirms proper warning device function. Other actions won't properly verify it.
What is the purpose of an anti-compounding valve?
Combine both circuits for extra force
Relieve excess reservoir pressure
Prevent rear axle brakes from applying if spring brakes are engaged
Increase governor cut-in pressure
An anti-compounding valve prevents air from being applied to the rear service brakes when spring brakes are already engaged, avoiding excessive brake force and potential lockup. It does not regulate tank pressure or circuit fusion. Proper use ensures balanced braking.
Which method correctly adjusts S-cam brakes?
Adjust until no pushrod movement
Adjust by feel without measurement
Adjust until pushrod moves about 1 inch at full release
Tighten until shoes drag
S-cam brakes require about one inch of pushrod travel when the brakes are released; this clearance prevents dragging while ensuring full application. Over-tightening causes drag, while zero clearance leads to overheating. Measurement tools should be used for accuracy.
Why is governor slack adjustment important in an air compressor?
Lubricates the compressor
Directly controls brake application
Reduces compressor noise
Ensures correct cut-in and cut-out pressures
Proper slack adjustment on the compressor governor ensures it signals the compressor at the correct lower and upper pressure limits, maintaining optimal system pressure. Incorrect slack can lead to over-pressurization or insufficient pressure. It does not lubricate or apply brakes.
After draining air tanks in a pre-trip inspection, what should you check next?
Fuel pressure gauge
Air build-up time to governor cut-out
Brake pedal travel only
Compressor oil level
Testing air build-up time ensures the compressor and governor function within specifications - typically under 45 seconds from 85 to 100 psi at idle. This verifies system readiness under load. Brake pedal travel alone doesn't assess air generation.
What is the maximum allowable build-up time from 85 to 100 psi at engine idle speed?
60 seconds
30 seconds
45 seconds
90 seconds
Regulations require air pressure to build from 85 to 100 psi in 45 seconds or less at idle speed, ensuring the compressor and system can maintain safe braking readiness. Longer times indicate leaks or compressor issues.
What should you do if air pressure is too low to release the spring brakes?
Use parking brake handle
Continue driving slowly
Stop and let the compressor build pressure
Shift to neutral and coast
If air pressure falls below the threshold needed to release spring brakes, you must stop and allow the compressor to build pressure. Continuing can leave you with locked brakes or unsafe handling. Parking brake handles won't override lack of air.
Which component automatically purges condensate from the air dryer?
Purge valve
Check valve
Anti-compounding valve
Governor
The purge valve on an air dryer opens at the end of each compressor unload cycle to expel moisture and contaminants. Governors control pressure, check valves direct flow, and anti-compounding valves prevent brake overlap. Proper purging maintains system health.
What role does the foot valve (brake pedal valve) play in an air brake system?
Controls air flow to service brakes
Regulates governor cut-in
Measures tank pressure
Drains air tanks
The foot valve, often called the brake pedal valve, regulates how much air is sent to the service brakes based on pedal position. It does not measure pressure, drain tanks, or affect the governor. Proper function ensures proportional braking.
What is a likely cause of uneven brake application on one side of a vehicle?
Properly inflated tires
Equal slack adjuster settings
Blocked air line or faulty valve
Balanced cargo load
A blocked air line or malfunctioning valve can prevent air from reaching one brake chamber, causing uneven braking force. Equal adjustments and balanced load are correct conditions, while tire inflation affects only traction, not air flow. Diagnosing leaks or obstructions resolves this.
Where should you see air pressure loss during full service brake application?
In the fuel line
At brake chambers and control valves
At the governor only
In the engine oil system
When service brakes are applied, air flows from the reservoirs through valves to the brake chambers, causing pressure drop primarily at those points. Fuel lines and oil systems are unrelated. Monitoring chamber pressure confirms proper actuation.
At what pressure range should a tractor protection valve close to protect the trailer circuit?
15 - 25 psi
20 - 45 psi
60 - 80 psi
50 - 70 psi
Tractor protection valves are designed to close and protect the trailer air supply when tractor pressure falls between 20 and 45 psi, preventing complete loss of braking capacity on the trailer. This autonomy is required by regulations.
What is the minimum required capacity of the tractor supply air reservoir on a combination vehicle?
100 gallons
60 gallons
40 gallons
80 gallons
Regulations specify that the tractor supply reservoir must hold at least 60 gallons of air to ensure sufficient capacity for trailer braking and system redundancy. Less capacity could lead to rapid pressure drop under heavy use.
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Study Outcomes

  1. Understand Core Components -

    Recognize what all air brake equipped vehicles have by identifying parts like brake drums, valves, and chambers to ensure proper system awareness.

  2. Apply Pressure Gauge Functions -

    Explain air brake pressure gauge function and interpret readings accurately for safe operation and maintenance of braking systems.

  3. Analyze Dual Air Brake Setups -

    Determine if your truck has a dual air brake system by assessing its configuration and understanding how two-line systems enhance safety.

  4. Perform Safety Checks -

    Conduct an air brake system safety check by following key inspection steps to identify leaks, pressure drops, and component wear.

  5. Evaluate Emergency Brake Readiness -

    Assess emergency brake components and scenarios to ensure vehicles maintain stopping power under system failures.

Cheat Sheet

  1. Pressurized Air Supply -

    All air brake equipped vehicles have an air compressor, air reservoirs, and pressure governor that maintain system pressure between 100 - 120 psi. A handy mnemonic is "C-R-G" (Compressor, Reservoir, Governor) to remember the core supply components. Proper maintenance, as outlined by FMCSA regulations, ensures reliable brake actuation and prevents moisture buildup.

  2. Air Brake Pressure Gauge Function -

    The air brake pressure gauge monitors system pressure in primary and secondary circuits and warns drivers when pressure drops below 60 psi. For example, a dual-needle gauge shows both circuits in real time to help detect leaks early. Regular checks per SAE International guidelines prevent brake fade and ensure accurate readings on every trip.

  3. Dual Air Brake System Safety -

    If your truck has a dual air brake system, it splits into two independent circuits - one for the front axle and one for the rear. A failure in one circuit won't disable the other, offering redundancy; remember "Two Circuits, Twice the Safety." Periodic leak-down tests, as recommended by NHTSA, verify each side holds pressure for at least two minutes.

  4. Slack Adjusters and Brake Drum Clearance -

    All air brake equipped vehicles have automatic or manual slack adjusters to maintain proper shoe-to-drum clearance, typically around 0.04 in. Use a ruler to check that pushrod stroke doesn't exceed 1 in during a static test, per manufacturer specs. This simple check, endorsed by technical bulletins from industry leaders, ensures responsive braking.

  5. Spring/Emergency Brake Mechanism -

    The spring brake chambers act as fail-safes, applying brakes mechanically if system pressure falls below 20 - 45 psi. To test, build pressure then shut off the engine and watch for spring engagement - verify it applies within the required pressure range. This feature, governed by FMCSA Part 393, safeguards against total brake loss.

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