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Quizzes > Science

CDL Air Brakes Quiz: Spring Brake Power

Quick, free check of your CDL spring brakes knowledge. Instant results.

Editorial: Review CompletedCreated By: Seth FilthUpdated Aug 28, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
paper art illustration of air brake system parts and control icons on sky blue background promoting free CDL air brake quiz

This quiz helps you check your understanding of spring brake power and key air-brake controls. Practice pedal use, control checks, and parking brake rules to spot gaps before the exam. For a quick refresh, see the air brake system test and how to supply air to trailer tanks.

What is the primary function of spring brakes in an air brake system?
To maintain air pressure in the tanks
To regulate brake lining temperature
To modulate the service braking
To apply parking and emergency brakes
Spring brakes use powerful coil springs to mechanically apply the brakes when air pressure is released or lost, typically for parking or emergency stopping. This design ensures that brakes engage even if the air system fails. They are not involved in normal service brake modulation, which relies on air pressure alone.
In a spring brake chamber, what component uses stored energy to apply the brakes when air pressure is lost?
Coil spring
Diaphragm
Brake adjuster
Brake shoe
A heavy-duty coil spring inside the spring brake chamber stores mechanical energy. If air pressure drops, that spring expands and pushes the pushrod to apply the brakes. The diaphragm and brake adjuster are pneumatic components for service braking.
At what approximate air pressure do spring brakes begin to release in most commercial vehicles?
80 psi
60 psi
150 psi
100 psi
Most heavy vehicles require around 90 to 120 psi to compress the spring fully and release the spring brakes, with 100 psi being a typical nominal value. Below that threshold, springs push the brakes on. This ensures sufficient pneumatic force to counteract the spring.
Which control in the cab is used to apply the spring brakes for parking?
Modulating hand control
Red emergency control valve
Service brake pedal
Yellow parking control valve
The yellow parking control knob (also called tractor parking valve) is pulled out to vent air from the spring brake chambers, allowing the springs to apply the brakes. The red valve handles trailer emergency lines, and the service pedal only controls service brakes.
What happens when the air lines to the spring brake chamber lose pressure?
Emergency lights activate
Service brake pressure increases
Spring brakes apply
Spring brakes release
Loss of air pressure in the spring brake chamber allows the powerful internal spring to expand, mechanically applying the brakes. This fail-safe action is crucial for preventing uncontrolled vehicle movement. It does not affect service brake pressure or lighting.
What is another name commonly used for spring brakes due to their function when air pressure is lost?
Emergency brakes
Service brakes
Retarder brakes
Secondary brakes
Spring brakes act automatically when air pressure drops, performing the role of emergency brakes. They also serve as parking brakes, but the term 'emergency brakes' highlights their fail-safe function during air system failures.
Where are spring brakes typically located on a commercial vehicle?
Brake chamber
Brake drum
Slack adjuster
Air compressor
Spring brake assemblies are integrated into brake chambers at each wheel end. The chamber houses both the service diaphragm and the mechanical spring for parking/emergency braking. Neither the compressor nor the slack adjuster contain spring brakes.
Why should spring brakes never be used while the vehicle is in motion except during an emergency?
They reduce air supply to the engine
They can cause wheel lockup and skidding
They save fuel at high speeds
They calibrate the brake lining
Applying spring brakes while moving forcibly engages the wheel brakes without modulation, often causing wheels to lock, skid, or even jackknife. They should only be used as a last resort in emergencies. Normal stopping relies on the service brake system.
During pre-trip inspection, how can you test that spring brakes apply properly?
Brake at top speed briefly
Pump the service pedal rapidly
Chock wheels and drain air pressure
Use the trailer hand valve at 30 mph
To confirm spring brake application, park on level ground, chock wheels, shut off the engine, and drain system air pressures. The park spring brakes should engage fully when tanks drop below the emergency application pressure. This test ensures both parking and emergency brakes work.
When parking a loaded tractor-trailer on a hill, what additional step ensures the spring brakes hold?
Block the service brake reservoir
Rotate the gladhand couplers
Chock the wheels
Reduce spring brake pressure
Even with spring brakes applied, wheel chocks provide mechanical backup to prevent rolling on slopes. Chocking is a required safety step on any grade. It also protects against spring brake failure or leakage.
How do you release the spring brakes on most heavy vehicles after parking?
Step on the service brake pedal
Adjust the slack adjuster
Pull out the red valve
Push the yellow knob in
Pushing in the yellow parking control valve allows air to flow back into the spring brake chambers, compressing the springs and releasing the brakes. Pulling the red valve controls trailer emergency brakes, and service pedal or adjusters do not release park springs.
What can cause a spring brake to become 'caged' and unusable?
Accumulation of moisture in the lines
Loose pushrod connections
Excessive use of a caging tool without air pressure
Worn brake linings
Mechanics use a caging tool to compress the spring into a held position for service work. If done without following proper air release procedures, the spring can become stuck in a compressed (caged) state and fail to apply. Moisture and wear do not cause caging.
What is the maximum allowable air leak rate per minute at service brake application according to FMCSA regulations?
1 psi per minute
10 psi per minute
5 psi per minute
2 psi per minute
For a single vehicle, the maximum air loss when the service brake is applied and held should not exceed 2 psi in one minute. This ensures system integrity under load. Combination vehicles allow up to 3 psi per minute.
Which component ensures that spring brakes apply automatically in the event of a trailer air supply line failure?
Air governor
Tractor protection valve
Relay emergency valve
Foot brake valve
The tractor protection valve closes the tractor's air supply to the trailer when the emergency line pressure drops, causing the trailer's spring brakes to apply. This prevents a runaway trailer if the line breaks. The relay valve and governor serve other pneumatic functions.
Under what condition will the spring brakes activate before the service brakes during an emergency braking situation?
If ABS is active
If service brake pressure exceeds governor cut-in
If air supply to the spring chamber is lost first
If slack adjuster is over-stroked
Spring brakes apply whenever air pressure to their chamber falls below the emergency application threshold, regardless of service brake application. If that supply is lost due to a line rupture or valve closure, the spring brakes engage immediately. Service brakes rely on air pressure and will not act first.
In a tractor-trailer combination, which cab control specifically activates only the trailer's spring brakes?
Electric hand switch
Foot service brake valve
Yellow parking control knob
Red emergency control knob
The red emergency control knob vents air from the trailer emergency line, causing the trailer spring brakes to apply. The yellow knob controls the parking springs on both tractor and trailer. The service pedal only applies service brakes.
How does an Anti-Lock Braking System (ABS) interact with spring brakes during braking?
ABS increases spring brake force
ABS alternates between spring and service brakes
ABS modulates only the service brake chambers
ABS disables spring brakes
ABS sensors and modulators control only the pneumatic service brake circuit to prevent wheel lockup during dynamic braking. Spring brakes are purely mechanical once applied and are not modulated by the ABS system.
How does brake chamber size affect spring brake application force?
Smaller chambers produce more force
Inverse to air pressure
Larger chambers produce more force
Chamber size has no effect
With the same air pressure, a larger chamber area multiplies the force output of both diaphragm and spring components. This provides greater mechanical advantage for applying the brakes. Smaller chambers yield less force.
What is the purpose of a dual parking control valve in some air brake systems?
To separately operate tractor and trailer spring brakes
To regulate governor cut-out pressure
To modulate service and emergency brakes together
To test ABS functionality
A dual parking control valve allows the driver to apply parking springs on the tractor separately from the trailer. This feature helps in certain coupling and parking procedures. It does not alter service braking or ABS systems.
During a 'bleed down' test on spring brakes, how long should it take for system pressure to drop from service to emergency application pressure in a single vehicle?
30 seconds
45 seconds
60 seconds
15 seconds
Federal regulations require that when the service brake is held applied, pressure should bleed down from full service to emergency application pressure within 45 seconds in a single vehicle. This ensures adequate sealing and system health.
What can happen if a brake chamber pushrod stroke exceeds the maximum specified length?
Service brakes lock out
Brakes automatically self-adjust
Spring brakes engage at lower pressure
Braking efficiency decreases and damage may occur
An over-extended pushrod indicates worn or maladjusted brakes, reducing mechanical advantage and causing poor braking performance, heat buildup, and potential chamber or slack adjuster damage. Proper adjustment keeps stroke within limits.
What is the effect of cross-connecting the service and parking brake circuits?
It causes spring brakes to apply when using service brakes
It improves brake response time
It has no effect on braking
It disables ABS
Cross-connections allow air intended for one circuit to vent through the other, causing unintended spring brake application whenever the service brakes are used. This compromises brake control and can be dangerous.
How do you calculate the theoretical spring force output in a brake chamber given a spring constant k and compression distance x?
F = k × x
F = k / x
F = k + x
F = ½ k x²
Hooke's Law states that the force exerted by a spring is proportional to its displacement: F = k × x. Here, k is the spring constant and x is the compression (or extension) distance. This linear relationship applies to the brake chamber's coil spring.
When selecting a spring brake chamber for a heavy-duty vehicle, which factor ensures the parking brake can hold at least 20% of the axle weight?
Diaphragm material composition
Brake drum diameter
Chamber spring force rating relative to axle load
Maximum pushrod length
Manufacturers specify spring brake chambers by spring force output. To meet a 20% holding requirement, choose a chamber whose static spring force exceeds 20% of the axle's weight. Diaphragm, rod length, or drum size are secondary considerations.
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Study Outcomes

  1. Understand the Braking Power of Spring Brakes -

    Learn how the braking power of the spring brakes contributes to stopping force and vehicle safety under different load and pressure conditions.

  2. Analyze the Role of the Brake Pedal in an Air Brake System -

    Examine how the brake pedal in an air brake system regulates air pressure and initiates braking action for controlled stops.

  3. Apply Procedures If the Spring Brakes Are On -

    Master the correct steps to take if the spring brakes are on you should engage or release them safely and effectively.

  4. Identify How Spring Brakes Are Held Back During Normal Driving -

    Recognize the air pressure mechanisms that during normal driving spring brakes are usually held back by to prevent unintended engagement.

  5. Interpret Maximum Leakage Rates for CDL Air Brake Systems -

    Evaluate what are the maximum leakage rates CDL regulations allow to ensure your air brake system meets safety standards.

  6. Evaluate Inspection and Maintenance Best Practices -

    Apply real-world maintenance tips and inspection routines to keep your spring brakes and air brake system in peak operating condition.

Cheat Sheet

  1. Spring Brake Force Mechanics -

    Understanding the braking power of the spring brakes is essential: these springs store mechanical energy that translates into high clamping force when air pressure falls below 20 - 45 psi. Think of them like powerful mousetraps held back by air - once released, they deliver consistent emergency and parking brake force without hydraulic or air assistance.

  2. Brake Pedal Valve Control -

    The brake pedal in an air brake system operates the foot valve (or treadle valve) that meters air to the service brake chambers based on pedal travel. Remember the simple motto "push to apply, release to exhaust" - this helps you visualize airflow direction when practicing brake modulation.

  3. Spring Brake Retention -

    During normal driving spring brakes are usually held back by air pressure in the service brake chamber, preventing the internal springs from expanding. A mnemonic "AIR Holds Springs" can remind you that sufficient system pressure (usually above 60 - 90 psi) keeps the parking/emergency springs compressed during motion.

  4. Emergency Spring Brake Engagement -

    If the spring brakes are on you should immediately check the air pressure gauge, pull to the side, and stop safely: losing pressure below the minimum will automatically release air and allow the springs to apply. After stopping, troubleshoot leaks or compressor failure before resetting the system to avoid repeat engagements.

  5. CDL Leakage Rate Standards -

    When taking a pre-trip check, recall what are the maximum leakage rates CDL mandates: with the engine off and brakes applied, single vehicles can lose up to 3 psi per minute, combinations up to 4 psi. With the engine running at idle, the acceptable leakage drops to 2 psi per minute for both single and combination rigs.

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