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What is the purpose of the air compressor governor?
- It opens the supply lines to the brake chambers
- It controls when the air compressor pumps air into the storage tanks ✓
- It releases excess air from the tanks at all times
- It engages the parking brake automatically when air pressure drops
The governor controls when the air compressor will pump air into the storage tanks. When tank pressure rises to the 'cut-out' level (around 125 psi), the governor stops the compressor from pumping. When pressure falls to the 'cut-in' level (around 100 psi), the governor allows the compressor to star…
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What does the term 'wet tank' mean in an air brake system?
- A tank used to store water for the cooling system
- The first storage tank the air compressor pumps to, which collects moisture and oil ✓
- Any storage tank that has failed its annual inspection
- A tank that has been intentionally filled with water for testing
The 'wet tank' (also called the supply tank or supply reservoir) is the first air tank that the compressor pumps into. It collects moisture and oil that are picked up during compression of the atmospheric air. Air contains water vapor, and compression causes some of that vapor to condense into liqui…
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What is the purpose of the safety valve installed in the supply (wet) tank?
- To release air from the system when the engine is shut off
- To protect the system from too much pressure if the governor fails ✓
- To bleed air slowly during a long descent
- To activate the spring brakes when needed
The safety valve is a spring-loaded valve in the supply (wet) tank set to open at about 150 psi. Its job is to protect the air brake system from over-pressurization in case the governor fails to cut out the compressor at the normal pressure (around 125 psi). If you hear the safety valve releasing ai…
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What is the function of slack adjusters in an air brake system?
- They reduce the amount of air the compressor must produce
- They take up the slack as brake shoes and drums wear, maintaining proper brake stroke ✓
- They allow the driver to manually loosen the brakes when stuck
- They reduce noise from the air brake system
Slack adjusters connect the brake chamber push rod to the brake camshaft. Their job is to take up the slack that develops as brake linings and drums wear, keeping the brake stroke (the distance the push rod must travel to apply the brakes) within specification. If brake stroke becomes too long, the …
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What is the purpose of spring brakes on a commercial vehicle?
- To provide additional braking on hills
- To act as the parking brake and as an emergency brake if air pressure is lost ✓
- To replace the regular service brakes on light loads
- To compress the air in the storage tanks
Spring brakes serve two functions: they are the parking brake (engaged when the driver pulls the yellow control), and they are the emergency brake that automatically applies if the air pressure in the brake system drops below a safe level (typically 20–45 psi). Spring brakes are powered by mechanica…
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What is the advantage of a dual air brake system?
- It uses half as much air
- It has two separate air brake systems, so if one fails the other can still stop the vehicle ✓
- It only requires inspection every other day
- It allows the driver to disable the brakes for towing
A dual air brake system has two completely separate air brake systems on the same vehicle, typically split between the front axle and the rear axles. Each system has its own air tanks, lines, and controls. If one system fails (a broken line, for example, or a leak), the other system continues to fun…
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At what air pressure does the low-pressure warning device come on?
- Before pressure drops below 60 psi ✓
- Below 100 psi
- Below 20 psi only
- Only when the engine is off
The low air pressure warning device must come on before pressure in either dual brake system drops below 60 psi. In most vehicles it activates at 55–60 psi. The warning may be a buzzer, a flashing light, a wig-wag arm that drops into view, or some combination of these. When the warning comes on whil…
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During the air brake inspection, what should air pressure build-up be?
- From 50 to 90 psi within 3 minutes (typical truck) ✓
- From 50 to 90 psi within 30 seconds
- From 0 to 150 psi within 1 minute
- Build-up time does not matter as long as the brakes work
For a typical truck with a dual air brake system, air pressure should build from 50 psi to 90 psi within 3 minutes. Slow build-up suggests a compressor problem, a major leak, or other issues that need attention before driving. The exact specification varies by vehicle (some require build-up within 2…
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How should you test the air leakage rate in a parked truck?
- With the engine running and your foot off the brake
- With the engine off and the brakes released, no more than 2 psi loss per minute for a straight truck or 3 psi for a combination vehicle ✓
- By listening for hissing sounds near the wheels
- Only as part of an annual inspection
To check air leakage: bring air pressure to governor cut-out (around 125 psi), turn off the engine, chock the wheels, release the parking brake, time one minute, and observe the air pressure drop. The maximum acceptable leakage rate is 2 psi per minute for a straight (single) vehicle and 3 psi per m…
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How do you check that the air pressure warning signal activates correctly?
- Drive the vehicle until the pressure drops naturally
- With the engine off, turn the key on and pump the brake pedal until pressure drops; the warning should come on before pressure falls below 60 psi ✓
- Disconnect the air supply hose and watch the gauge
- Wait until the next governor cycle and listen for a beep
To test the low-pressure warning: with the engine off and the key in the on position, pump the brake pedal repeatedly to bleed air out of the system. Watch the air pressure gauge. The warning signal (buzzer, light, or wig-wag) must come on before pressure drops below 60 psi. If it does not, the warn…
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When checking the spring brakes activate at the correct pressure, what should happen?
- The parking brake should release when air pressure builds above 60 psi
- The spring brakes (parking brake) should apply automatically when air pressure drops to between 20 and 45 psi ✓
- The service brake pedal should depress automatically
- The compressor should engage immediately
To check spring brake activation: continue bleeding air out of the system (engine off, key on, brake pedal pumped) past the point where the low-air warning activated. The parking brake control should pop out automatically (or the brakes should otherwise apply) when air pressure drops to between 20 a…
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What is the proper way to apply the air brakes during normal driving?
- Press the pedal as hard as possible to ensure full braking
- Apply steady, light pressure and release when speed has dropped enough ✓
- Pump the brake repeatedly
- Hold the parking brake control to slow down
Normal braking uses steady, light pedal pressure. Push the pedal down just enough to feel the truck slow, hold steady pressure, and release smoothly when speed has dropped to your target. Heavy braking should be reserved for emergencies and stops where heavy braking is required. Pumping is not neces…
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What is brake fade, and what causes it?
- A normal cycle of the air compressor
- Loss of braking effectiveness due to overheated brakes on long descents ✓
- Reduced air pressure on cold mornings
- The brakes adjusting themselves after high-speed use
Brake fade is the loss of stopping power that occurs when brakes overheat. Friction between the brake linings and drums or rotors generates heat, and as the brakes get hotter, they lose grip. On a long downgrade where the brakes are applied continuously to control speed, heat builds up faster than i…
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On a long downgrade, what gear should you select?
- The same gear you used to climb the hill
- A gear one or two lower than what you would use to climb the hill (for older vehicles); newer engines may allow slightly higher ✓
- Always neutral to save fuel
- The highest gear that the engine can handle
The traditional rule of thumb for older trucks is to descend in a gear lower than the gear used to climb the same grade. Modern trucks with more powerful engines and engine brakes may allow descending in a slightly higher gear, but the principle is the same: select a gear that lets engine braking an…
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What is the proper procedure if your brakes start to fail on a downgrade?
- Pump the brakes harder to restore pressure
- Look for an escape ramp and use it immediately ✓
- Steer for the shoulder and brake to a stop
- Use the parking brake to slow down
If your brakes start to fade or fail on a downgrade, look for a runaway truck escape ramp and use it without hesitation. Escape ramps are loose-gravel or sand inclines specifically engineered to stop a runaway truck safely. The longer you wait to commit, the faster you will be going, and the harder …
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What does it mean if the spring brakes apply automatically while you are driving?
- The truck is operating normally
- Air pressure has dropped below the safe threshold; you must stop immediately ✓
- The compressor has reached cut-out pressure
- You have been driving too fast
Automatic application of the spring brakes during driving means air pressure has dropped below 20–45 psi, which is well below the safe operating range. This is a serious emergency: the parking brake is now applied at full force, which can lock the rear wheels and cause a skid or jackknife. The cause…
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What is a 'stop light switch' in an air brake system?
- A device that controls the brakes for traffic signal compliance
- An electric switch that turns on the brake lights when air pressure is applied to the service brakes ✓
- A manual override for the parking brake
- A backup for the headlights
The stop light switch is an electric switch operated by air pressure in the service brake line. When you press the brake pedal, air pressure rises in the service brake line, the switch closes, and the brake lights at the rear of the vehicle turn on. This is the same function as in a passenger car, b…
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What does the front brake limiting valve do?
- Prevents the front brakes from working at high speeds
- On older vehicles, reduces braking force to the front axle on slippery roads when set to 'slippery' ✓
- Limits the number of times the brake pedal can be used
- Disconnects the brakes when towing
Older trucks (made before 1975 or so) sometimes had a front brake limiting valve that the driver could set to 'normal' or 'slippery'. When set to 'slippery', the valve reduced air pressure to the front brakes, the idea being to prevent the front wheels from locking on slick surfaces and losing steer…
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Why is it a bad idea to drive a vehicle with anti-lock brakes (ABS) as if it does not have ABS?
- ABS only works when the driver applies brakes lightly
- ABS provides automatic anti-skid help only during firm braking; failing to brake firmly defeats the system ✓
- ABS requires the driver to pump the brakes manually
- ABS works in reverse — the driver should brake gently to activate it
ABS provides anti-skid protection only when the driver applies the brakes hard enough that the wheels would otherwise lock up. During firm braking, the ABS system rapidly releases and reapplies brake pressure dozens of times per second to keep the wheels rolling at the threshold of grip, preserving …
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How often should you drain the air tanks?
- Only at annual inspection
- Daily, or whenever an automatic drain valve is unavailable; more often in humid or freezing conditions ✓
- Once a week
- Only when the tank pressure exceeds 150 psi
Air tanks should be drained every day. Most modern trucks have automatic drain valves that release water and oil periodically without driver action, but drivers should still verify these are functioning. If automatic drains are not present or have failed, drain manually by opening the petcock at the…
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What is the purpose of the alcohol evaporator in some air brake systems?
- To add fuel to the engine in cold weather
- To put alcohol into the air system in winter, lowering the freezing point of any water in the lines ✓
- To clean the air filter
- To evaporate water from the engine coolant
Some air brake systems include an alcohol evaporator, especially in cold climates. The evaporator slowly introduces methyl alcohol into the air supply during winter operation. The alcohol mixes with any moisture in the system and lowers its freezing point, preventing ice from blocking lines or valve…
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What is the function of the air dryer?
- To cool the engine
- To remove water and oil from the compressed air before it reaches the tanks ✓
- To eliminate the need for air tanks
- To increase air pressure beyond 150 psi
The air dryer sits between the compressor and the storage tanks. It removes water vapor and oil mist from the compressed air before that air enters the system. By drying the air, it reduces the risk of water freezing in lines, prevents corrosion in tanks and valves, and extends the life of brake com…
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In a dual air brake system, what is the maximum allowable air loss rate with brakes applied for a combination vehicle?
- 1 psi per minute
- 2 psi per minute
- 4 psi per minute ✓
- 10 psi per minute
The maximum allowable air leakage rate for a combination vehicle (tractor-trailer) with brakes applied is 4 psi per minute. For a single vehicle (straight truck) with brakes applied, it is 3 psi per minute. With brakes released (no application), the limits are tighter: 2 psi per minute for a single …
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How does total stopping distance for an air-braked vehicle compare to a hydraulic-braked passenger car at the same speed?
- Air brakes stop in about half the distance
- Air brakes take longer because air takes time to travel through the lines and act on the brakes ✓
- They are identical
- Air brakes stop instantly with no lag
Air-braked vehicles have an additional component to stopping distance that hydraulic-braked vehicles do not: brake lag. When you press the brake pedal, air pressure must travel from the valve through the lines to each brake chamber. This takes about half a second — short, but at 55 mph the truck tra…
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What is the proper way to make an emergency stop with non-ABS air brakes?
- Press the pedal as hard as possible and hold
- Use 'controlled braking' or 'stab braking' to prevent wheel lockup while applying maximum braking ✓
- Apply the parking brake as well
- Pump the brakes rapidly
On a vehicle without ABS, emergency braking requires controlled technique to prevent wheels from locking. Two methods: 'Controlled braking' is steady firm pressure just below the lockup point; if a wheel locks, release the pedal briefly and reapply. 'Stab braking' is full application until lockup oc…
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What is the function of the air compressor governor in an air brake system?
- It powers the steering system
- It controls when the air compressor loads and unloads — cutting out (stopping compression) at approximately 120-125 psi and cutting in (resuming compression) at approximately 100 psi ✓
- It regulates fuel injection timing
- It monitors wheel speed for ABS
The AIR COMPRESSOR GOVERNOR is the regulating valve that controls the air compressor's loading and unloading cycles to maintain safe operating pressure in the air tanks. OPERATION: CUT-OUT PRESSURE — when system pressure reaches approximately 120-125 psi (the governor's upper limit), the governor si…
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What is the function of the supply (wet) tank in an air brake system?
- It stores the final pressurized air used directly by the brakes
- It is the first tank that receives compressed air from the compressor — it collects oil, water, and contaminants through a drain valve, protecting downstream system components ✓
- It only activates in emergencies
- It controls brake chamber timing
The SUPPLY TANK (also called WET TANK or primary reservoir) is the first air tank in the air brake system. It sits immediately downstream from the air compressor and has specific functions: RECEIVES COMPRESSED AIR FIRST: Air from the compressor enters the supply tank before going anywhere else in th…
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What is the 'fanning' or 'snubbing' braking technique and when should it be used?
- Rapidly pumping the brakes repeatedly like a car with drum brakes
- Applying brakes firmly until speed decreases by approximately 5 mph, then releasing brakes completely for a brief recovery, then reapplying — used on long grades to allow brake cooling between applications ✓
- Using only engine braking without any friction brakes
- Applying trailer brakes independently of service brakes
SNUBBING (also called CONTROLLED BRAKING or brake fanning) is an approved technique for descending long grades. HOW IT WORKS: Apply service brakes firmly until vehicle speed decreases by approximately 5 mph (for example, from 30 mph to 25 mph); COMPLETELY RELEASE brakes — allowing drums and pads to …
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During the air brake pre-trip inspection, why must the driver check that spring brakes automatically apply when air pressure falls?
- To test if the air compressor works
- Spring brakes are the safety backup — if service air is lost while driving, spring brakes must automatically apply to stop the vehicle; a spring brake that fails to apply at low pressure is a critical safety failure ✓
- Spring brakes are not tested in pre-trip
- To verify the speedometer works
SPRING BRAKE AUTOMATIC APPLICATION TEST is a critical safety check because spring brakes serve as both the parking brake AND the emergency backup braking system. WHAT SPRING BRAKES ARE: Spring brakes work on a fail-safe principle — they are held RELEASED by air pressure (typically 60-90 psi keeps th…
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In a dual air brake system, what happens if one of the two circuits develops a major air leak while driving?
- All braking is immediately lost
- The affected circuit loses pressure, but the other circuit remains functional — the system protection valve isolates the failure, and the remaining circuit provides braking on either the front or rear axles ✓
- Both circuits lose pressure simultaneously due to shared tanks
- The driver receives no warning and must rely on engine braking only
DUAL AIR BRAKE SYSTEM DESIGN provides redundancy specifically to prevent total brake failure from a single air line rupture. HOW DUAL SYSTEMS WORK: Modern CMVs use two completely separate circuits: the PRIMARY circuit (typically rear axle service brakes) and SECONDARY circuit (typically front axle s…
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At what air pressure should the low air pressure warning activate, and what should the driver do immediately when it does?
- It activates at 30 psi; continue to the nearest truck stop
- It activates at 60 psi or above; the driver should immediately slow down and pull off the road safely — spring brakes will automatically apply when pressure falls to approximately 20-45 psi ✓
- It activates at 100 psi; reduce speed by 10 mph
- It activates at 15 psi; only then should the driver stop
LOW AIR PRESSURE WARNING ACTIVATION: Per FMCSA regulations, the warning device (buzzer, light, or both) must activate before air pressure drops below 60 psi. Many systems activate at 60 psi exactly; others activate slightly higher. WHY 60 PSI IS THE TRIGGER: At 60 psi, the system is still above the …
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What is the role of the brake chambers in a pneumatic air brake system?
- They store compressed air as backup tanks
- They convert air pressure into mechanical force — when service air enters, a flexible diaphragm pushes a push rod that actuates the slack adjuster and camshaft to apply the brake shoes ✓
- They regulate the speed of air delivery
- They are the air compressor mounting points
BRAKE CHAMBERS (service chambers) are the air-to-mechanical energy conversion devices at each wheel. CONSTRUCTION: A cylindrical metal housing divided by a large rubber diaphragm; an inlet port for service air; a push rod connected through the center of one housing half; and a return spring to relea…
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What is the proper use of the foot brake (treadle/foot valve) when bringing a loaded vehicle to a normal stop?
- Tap the brakes quickly three times to warn following drivers, then apply hard
- Apply brakes smoothly and progressively — press the pedal gradually to build pressure, maintain steady application, then ease off slightly just before stopping to avoid jolting the load and passengers ✓
- Apply maximum pressure immediately to reduce brake wear
- Use engine braking exclusively and avoid the foot brake for normal stops
SMOOTH PROGRESSIVE BRAKING is the professional standard for commercial vehicle stops. It contrasts with both panic-style hard braking and the 'tap-tap-hard' technique some drivers learn. THE TECHNIQUE: (1) BEGIN BRAKING EARLY — commercial vehicles need more stopping distance than cars; plan stops 2-…
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What is the acceptable air pressure loss rate during the static leak test with spring brakes released?
- 10 psi per minute
- Less than 3 psi in 1 minute for single vehicles; less than 4 psi in 1 minute for combination vehicles ✓
- 25 psi per minute
- No loss is acceptable at all
THE STATIC LEAK TEST evaluates the integrity of the entire air brake system — lines, fittings, chambers, valves — by measuring how much pressure is lost when the system is pressurized and static (no movement, no brake application). PROCEDURE: (1) Build air pressure to governor cut-out range (120-125…
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What is the purpose of the one-way check valves in an air brake system with multiple tanks?
- They prevent drivers from releasing brakes manually
- They allow air to flow in only one direction — preventing air from backing up from one tank to another and allowing each tank to maintain its pressure independently ✓
- They regulate airflow speed between tanks
- They replace the safety relief valve function
ONE-WAY CHECK VALVES (or check valves) are installed at various points in the air brake system to enforce unidirectional airflow. PRIMARY PURPOSE — TANK ISOLATION: When air flows from the compressor through the supply tank toward the primary and secondary service tanks, check valves prevent air from…
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What is the purpose of having a dual air brake system on a commercial motor vehicle?
- To provide more braking force
- To provide a backup braking system — if the primary circuit loses pressure, the secondary circuit maintains braking ability; the vehicle is designed to stop safely even if one circuit fails completely ✓
- To allow braking at higher speeds
- Dual systems are only required on vehicles over 80,000 lbs
DUAL AIR BRAKE SYSTEM SAFETY: A single-circuit air brake system that fails can result in complete brake loss. The dual system solves this by dividing brake application across two independent circuits: PRIMARY CIRCUIT: Typically operates rear drive axle brakes; SECONDARY CIRCUIT: Typically operates f…
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What is 'brake fade' and when does it most commonly occur?
- When brakes become sharper over time
- When brakes lose effectiveness due to heat buildup — most commonly on long, steep downgrades when the driver rides the service brakes continuously instead of using proper engine braking technique; overheated drums/rotors and brake lining glaze can reduce braking force to near zero ✓
- A mechanical failure unrelated to heat
- Only on vehicles with hydraulic brakes
BRAKE FADE in air-brake vehicles primarily results from thermal degradation: CAUSE: Continuous service brake application on a downgrade generates heat faster than drums/rotors can dissipate it; when drum/rotor temperature exceeds the lining's thermal rating, the lining material partially volatilizes…
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What is the purpose of the air compressor in an air brake system?
- To cool the brakes
- To pump air and build up pressure in the air storage tanks (reservoirs) that operate the brakes ✓
- To apply the brakes directly
- To steer the vehicle
The AIR COMPRESSOR pumps air to build up and maintain pressure in the air storage tanks (reservoirs). ASVAB/CDL air brake systems use compressed air to apply the brakes. The compressor is driven by the engine; the GOVERNOR controls when the compressor pumps air (cutting in/out to maintain pressure, …
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What does the air brake governor control?
- The steering
- When the air compressor pumps air into the reservoirs (cut-in and cut-out pressures) ✓
- The engine speed
- The headlights
The GOVERNOR controls when the air compressor PUMPS air into the reservoirs — it stops the compressor from pumping (CUT-OUT) when pressure reaches the maximum (around 125 psi) and starts it again (CUT-IN) when pressure drops to the minimum (around 100 psi). ASVAB/CDL air brakes. This maintains air p…
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At what air pressure does the low air pressure warning signal typically activate?
- Below 60 psi (usually around 60 psi or below) ✓
- At 125 psi
- At 200 psi
- It never activates
The LOW AIR PRESSURE WARNING signal must activate before pressure drops below 60 psi (typically activating at around 60 psi or below) — alerting the driver that air pressure is getting dangerously low. CDL air brakes. The warning may be a light, buzzer, and/or a 'wig-wag' device. If pressure keeps d…
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During an air brake check, you should know that air pressure should build from 85 to 100 psi within what timeframe?
- Within 45 seconds
- Within about 45 seconds for dual air systems (build-up rate test) ✓
- Within 10 minutes
- It doesn't matter
In a typical AIR PRESSURE BUILD-UP test for dual air systems, pressure should build from 85 to 100 psi within about 45 seconds. CDL air brakes pre-trip. This checks that the compressor is building air at an adequate rate. If pressure builds too slowly, the system may not maintain adequate air during…
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What are spring brakes, and how do they work?
- Brakes made of springs that bounce
- Emergency/parking brakes held in the 'off' position by air pressure; if air pressure is lost, the springs apply the brakes automatically (a fail-safe) ✓
- Brakes that only work in spring weather
- Brakes that increase speed
SPRING BRAKES are the emergency and parking brakes. They are held in the 'OFF' (released) position by AIR PRESSURE; powerful springs are compressed by the air. If air pressure is LOST (a leak or failure), the springs are released and AUTOMATICALLY APPLY the brakes — a FAIL-SAFE design. CDL air brake…
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Why should you drain the air tanks (reservoirs) regularly?
- To make the truck lighter
- To remove water and oil/sludge that accumulate and can damage the system or freeze in cold weather ✓
- To add more air
- It is unnecessary
Air tanks must be DRAINED regularly to remove WATER and oil/sludge that accumulate (the compressor pumps in air that contains moisture, which condenses in the tanks). CDL air brakes. Accumulated water can: reduce braking effectiveness, damage the system, and FREEZE in cold weather (causing brake fai…
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What is 'brake fade' and what causes it?
- Brakes changing color
- Loss of braking power caused by overheating the brakes (often from continuous/excessive use on long downgrades) ✓
- Brakes that work better when hot
- A type of brake fluid
BRAKE FADE is the LOSS of braking power caused by OVERHEATING the brakes — typically from continuous or excessive brake use, especially on long, steep DOWNGRADES. CDL air brakes. As brakes overheat, they lose their ability to create friction, so they become less effective (or fail). PREVENTION: use …
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When braking with air brakes during normal driving, what is the recommended technique?
- Pump the brakes rapidly
- Apply steady, controlled pressure (with modern ABS, do not pump); for non-ABS, brake firmly without locking the wheels ✓
- Slam the brakes hard always
- Never use the brakes
For NORMAL stops with air brakes, apply STEADY, CONTROLLED pressure. With ABS (Antilock Braking System), do NOT pump the brakes — apply firm, steady pressure and let the ABS prevent lockup. For older non-ABS systems, brake firmly without locking the wheels (controlled braking) or use 'stab braking' …
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What is the function of the brake chambers in an air brake system?
- To store fuel
- To convert air pressure into the mechanical force that applies the brakes at each wheel ✓
- To cool the engine
- To inflate the tires
BRAKE CHAMBERS convert AIR PRESSURE into the MECHANICAL FORCE that applies the brakes at each wheel. CDL air brakes. When the driver presses the brake pedal, air pressure enters the brake chamber, pushing a diaphragm and pushrod, which moves the SLACK ADJUSTER and turns the cam (or wedge) to press t…
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During a static air leakage test (engine off, brakes released), how much air loss per minute is generally acceptable for a single vehicle?
- Up to 2 psi per minute
- No more than about 2 psi per minute for a single vehicle (3 psi for a combination); excessive leakage indicates a problem ✓
- Up to 50 psi per minute
- Any amount is fine
In a STATIC AIR LEAKAGE test (engine off, system charged, brakes released), air loss should not exceed about 2 PSI per minute for a SINGLE vehicle (or about 3 psi per minute for a combination vehicle). CDL air brakes pre-trip. With the brakes APPLIED, the allowable rates are slightly higher (about 3…
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What does the term 'dual air brake system' mean?
- Two drivers operate the brakes
- Two separate air brake systems (typically one for the front, one for the rear) that share one set of controls, providing a backup if one system fails ✓
- Brakes that work twice as hard
- A system with no reservoirs
A DUAL AIR BRAKE SYSTEM has TWO SEPARATE air brake systems that use a single set of controls. CDL air brakes. Typically: one system operates the rear brakes, the other operates the front brakes; each has its own reservoirs, lines, and components. SAFETY/BACKUP: if one system fails (loses air), the O…
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On a long downhill grade with air brakes, what is the proper way to control speed?
- Ride the brakes continuously the whole way down
- Select a low gear before the descent and use engine braking as the primary control, supplemented by snub braking — to avoid overheating the brakes ✓
- Coast in neutral
- Brake only at the bottom
On a long downgrade, select a LOW GEAR BEFORE starting down and use ENGINE BRAKING as the primary speed control, supplemented by SNUB BRAKING (apply the brakes firmly to slow about 5 mph below your target speed, then release to let them cool; repeat). CDL air brakes. This prevents BRAKE FADE from co…
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If the low air pressure warning comes on while driving, what should the driver do?
- Speed up to reach the destination
- Stop safely as soon as possible and park the vehicle, because continued air loss will cause the spring brakes to apply (possibly suddenly) and the vehicle could become uncontrollable ✓
- Ignore it
- Pump the brakes repeatedly
If the LOW AIR PRESSURE WARNING activates while driving, STOP SAFELY as soon as possible and park. CDL air brakes. WHY: continued air loss will eventually cause the SPRING (emergency) BRAKES to apply automatically (around 20-45 psi) — possibly SUDDENLY and unexpectedly, which could cause loss of con…
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What is the purpose of the supply (wet) tank in an air brake system?
- To hold brake fluid
- To be the first reservoir that receives air from the compressor, allowing moisture and oil to collect before air goes to the service tanks ✓
- To store fuel
- To cool the brakes
The SUPPLY TANK (also called the 'wet tank') is the FIRST reservoir that receives compressed air from the compressor. Because air from the compressor carries MOISTURE and OIL, the wet tank lets these contaminants COLLECT/settle out before the air passes on to the service (dry) reservoirs that operat…
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What is the function of an air dryer in an air brake system?
- To warm the air
- To remove moisture and some oil from the compressed air before it reaches the air tanks, reducing water accumulation and freezing risk ✓
- To increase air pressure
- To cool the engine
An AIR DRYER removes MOISTURE (water vapor) and some oil from the compressed air BEFORE it reaches the air storage tanks. CDL air brakes. This reduces water accumulation in the system, which: prevents corrosion and damage; reduces the risk of FREEZING in cold weather (frozen moisture can block valve…
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What is a slack adjuster in an air brake system, and why does it need to be properly adjusted?
- A device to adjust the seat
- The component that connects the brake chamber pushrod to the brake cam; if out of adjustment, the brakes may not apply fully, reducing braking effectiveness ✓
- A fuel valve
- A type of tire
The SLACK ADJUSTER connects the brake chamber pushrod to the brake S-CAM (which spreads the brake shoes against the drum). It 'adjusts the slack' to keep the proper clearance between the brake shoes and drum as the linings wear. CDL air brakes. WHY ADJUSTMENT MATTERS: if the slack adjuster is OUT OF…
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During an applied air leakage test for a single vehicle, after fully applying the brakes, how much pressure drop per minute is generally acceptable?
- Up to 3 psi per minute ✓
- No more than about 3 psi per minute for a single vehicle (4 psi for a combination) with the brakes applied
- Up to 25 psi per minute
- Any amount is acceptable
In an APPLIED air leakage test (engine off, system charged, then fully APPLY and hold the brakes), the pressure drop should not exceed about 3 PSI per minute for a SINGLE vehicle (or about 4 psi per minute for a COMBINATION vehicle). CDL air brakes inspection. COMPARE: in the STATIC (brakes released…
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At approximately what air pressure do the spring (emergency/parking) brakes typically apply automatically if air pressure is lost?
- At 125 psi
- Between about 20 and 45 psi (varies by vehicle) ✓
- At 100 psi
- They never apply automatically
The SPRING (emergency/parking) BRAKES typically apply AUTOMATICALLY when air pressure drops to between about 20 and 45 PSI (the exact point varies by vehicle). CDL air brakes. This is the FAIL-SAFE: spring brakes are held OFF by air pressure, so when air is lost (a major leak or failure), the spring…
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What is 'brake lag' in an air brake system?
- The brakes never working
- The short delay (about half a second) between pressing the brake pedal and the brakes actually applying, because air takes time to flow through the lines ✓
- A type of brake fade
- The brakes applying too fast
BRAKE LAG is the short DELAY (about 1/2 second) between when the driver presses the brake pedal and when the brakes actually APPLY — because it takes time for the compressed AIR to travel through the brake lines to the brake chambers at each wheel. CDL air brakes. This is unique to air brakes (hydra…
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How should a driver test the parking (spring) brakes during a pre-trip inspection?
- Skip this test
- With the vehicle stopped, apply the parking brake, then gently try to pull forward in a low gear to confirm the parking brake holds the vehicle ✓
- Drive at high speed
- Honk the horn
To test the PARKING (spring) BRAKES: with the vehicle STOPPED and at a safe location, APPLY the parking brake (push in the parking brake knob), then release the service brake and gently try to move forward in a LOW gear — the parking brake should HOLD the vehicle (it should not move). CDL air brakes…
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What does the term 'application pressure' refer to in air brakes?
- The pressure in the tires
- The amount of air pressure applied to the brakes when the driver presses the brake pedal — more pedal pressure means more application pressure and more braking force ✓
- The pressure of the fuel
- The atmospheric pressure
APPLICATION PRESSURE refers to the amount of air pressure delivered to the brakes when the driver presses the brake pedal (the foot valve/treadle valve). CDL air brakes. The HARDER the driver presses the pedal, the MORE application pressure is sent to the brake chambers, producing MORE braking force…
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Why is it dangerous to 'fan' the brakes (rapidly pump them) repeatedly when descending a long grade with air brakes?
- It is not dangerous
- Repeatedly pumping the brakes uses up the compressed air faster than the compressor can replace it, which can deplete air pressure and lead to brake failure ✓
- It makes the truck faster
- It cools the brakes too much
FANNING (rapidly pumping) the air brakes repeatedly on a long downgrade is dangerous because each application USES compressed air, and rapid repeated pumping can use air FASTER than the compressor can replace it — depleting air pressure and potentially leading to LOSS OF BRAKES (and the spring brake…
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What is the foot valve (treadle valve) in an air brake system?
- A valve for the tires
- The brake pedal mechanism that controls how much air pressure is applied to the service brakes when the driver presses it ✓
- The fuel pedal
- A parking device
The FOOT VALVE (also called the treadle valve or brake pedal) is the mechanism the driver operates with their foot to apply the SERVICE BRAKES. CDL air brakes. Pressing the foot valve releases compressed air to the brake chambers — the harder you press, the more air pressure (application pressure) i…
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What should a driver check regarding the brake drums or rotors during an inspection?
- Only the color
- That they are free of cracks, not worn beyond limits, and free of oil, grease, or other contamination that could reduce braking ✓
- Nothing
- Only the tire pressure
During inspection, check the BRAKE DRUMS (or rotors): they should be free of CRACKS; not worn beyond safe limits (excessively worn/thin drums can crack or fail under heat); and FREE of oil, grease, or other contamination (which reduces friction and braking effectiveness). CDL air brakes inspection. …
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On a vehicle with anti-lock braking systems (ABS), what should the driver do during hard braking?
- Pump the brakes rapidly
- Apply firm, steady pressure and let the ABS prevent the wheels from locking up — do not pump the brakes ✓
- Take the foot off the brake
- Apply the parking brake
With ABS (Anti-lock Braking System), during hard/emergency braking the driver should apply FIRM, STEADY pressure and let the ABS do its job — the ABS automatically prevents the wheels from LOCKING UP (which would cause skidding and loss of steering control). CDL air brakes. DO NOT PUMP the brakes wi…
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What is the purpose of the safety valve (pressure relief valve) in the supply tank?
- To start the engine
- To protect the air system from over-pressurization by releasing air if the pressure gets too high (typically around 150 psi) ✓
- To apply the brakes
- To drain the tank
The SAFETY VALVE (pressure relief valve), usually installed in the supply (wet) tank, protects the air brake system from OVER-PRESSURIZATION. CDL air brakes. If the air pressure rises too high (typically the safety valve is set to release at around 150 psi) — for example, if the governor malfunction…
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What is the proper procedure for an emergency stop using air brakes if the vehicle does NOT have ABS?
- Lock up the brakes and hold
- Use controlled braking (apply firmly without locking the wheels) or stab braking (apply fully, release when wheels lock, reapply) to maintain control ✓
- Take the foot off the brake
- Only use the parking brake
For an emergency stop WITHOUT ABS, use either: CONTROLLED BRAKING — apply the brakes as hard as possible WITHOUT locking the wheels (locked wheels skid and you lose steering); or STAB BRAKING — apply the brakes fully, and when the wheels LOCK UP, RELEASE the brakes (until the wheels start rolling ag…
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As part of the air brake pre-trip check, why should a driver let air pressure build to the normal range before testing?
- It wastes time
- Because the brakes need adequate air pressure to operate properly, and several tests (low-air warning, governor cut-out, leakage) require the system to first be fully charged to the normal operating range ✓
- To cool the engine
- It is not necessary
During the air brake pre-trip, the driver should let air pressure BUILD to the normal operating range (about 100-125 psi) before performing the brake tests, because: the brakes need adequate air to operate properly; and several checks require a fully charged system first — the GOVERNOR CUT-OUT test …