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ME CDL Air Brakes study materials
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ME CDL Air Brakes practice questions

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Question 1Air brake system components and function

On a typical air brake system, what is the primary function of the air compressor?

  1. To release air from the spring brakes during normal operation
  2. To regulate the amount of air delivered to the brake chambers
  3. To build and maintain air pressure in the storage reservoirs Correct answer.
  4. To filter moisture and oil out of the air supply

Explanation

The compressor is the source of compressed air; it runs off the engine and pumps air into the reservoirs so pressure stays within the governor's cut-in and cut-out range. The governor, not the compressor, controls when the compressor loads and unloads. Filtering is done by the air dryer and the pressure delivered to the chambers is set by the relay and foot valves.

Question 2Air brake system components and function

What does the governor do on an air brake system?

  1. It drains accumulated moisture from the supply reservoir each cycle
  2. It limits the maximum pressure that can reach the brake chambers during a hard stop
  3. It controls when the compressor loads and unloads to hold reservoir pressure in a set range Correct answer.
  4. It automatically applies the spring brakes when pressure drops too low

Explanation

The governor senses reservoir pressure and signals the compressor to load (pump) at cut-in pressure and unload at cut-out pressure, typically around 100 and 125 psi. This keeps the system within its designed operating range. Limiting chamber pressure is the job of the foot valve and relay valve, and low-pressure protection comes from the spring brakes and warning devices.

Question 3Air brake system components and function

What is the purpose of the air dryer in a CDL air brake system?

  1. To supply air directly to the brake chambers during a panic stop
  2. To remove moisture and contaminants from the compressed air before it reaches the reservoirs Correct answer.
  3. To cool the compressed air so the compressor runs more efficiently
  4. To boost reservoir pressure above the governor cut-out point

Explanation

The air dryer uses a desiccant and often a coalescing filter to strip water vapor and oil from the air before it enters the wet tank and storage reservoirs. Keeping moisture out prevents corrosion and freeze-ups in valves and lines. It does not raise pressure or feed the chambers directly.

Question 4Air brake system components and function

A driver notices the low air pressure warning buzzer and light activate. At approximately what pressure must this warning come on?

  1. 90 psi or below
  2. 60 psi or below Correct answer.
  3. 120 psi or below
  4. 30 psi or below

Explanation

Federal standards require the low air pressure warning device to activate before or at 60 psi, giving the driver time to react before spring brakes apply. The warning must be both audible and visible. A typical system's governor cut-in near 100 psi is well above this threshold.

Question 5Air brake system components and function

What is the function of the slack adjuster?

  1. It converts the pushrod stroke into rotation of the brake camshaft to apply the brakes Correct answer.
  2. It automatically drains water from the supply reservoir
  3. It regulates air pressure delivered to the brake chamber
  4. It maintains a fixed air gap between the brake shoe and the drum

Explanation

The slack adjuster is a lever on the brake camshaft that the pushrod pushes against; its length multiplies pushrod force into torque that rotates the camshaft and expands the shoes. It also provides the adjustment that takes up wear. It does not control air pressure or drain reservoirs.

Question 6Air brake system components and function

What is the primary purpose of a spring brake (emergency/parking brake) chamber?

  1. To reduce air consumption by storing pressure for repeated stops
  2. To provide extra braking force during normal service brake applications
  3. To apply the brakes mechanically when air pressure is lost or when the parking brake is set Correct answer.
  4. To hold the pushrod retracted when the service brakes are released

Explanation

A spring brake uses a heavy coil spring to apply the brakes when air pressure is released, either by the parking brake valve or by a loss of system pressure. This gives automatic emergency and parking braking. During normal service braking, the spring is held off by air and the service chamber does the work.

Question 7Air brake system components and function

What does the safety valve on an air brake reservoir do?

  1. It releases air if reservoir pressure exceeds a preset limit to prevent damage Correct answer.
  2. It balances pressure between the primary and secondary circuits
  3. It automatically drains moisture from the reservoir
  4. It warns the driver when pressure falls below 60 psi

Explanation

The safety valve is a pressure relief device that opens if reservoir pressure rises above its setting, protecting tanks and components from over-pressurization. It is a backup to the governor. It does not drain moisture, warn of low pressure, or balance circuits.

Question 8Air brake system components and function

Why must a driver drain air tanks (reservoirs) regularly?

  1. To release the spring brakes before driving
  2. To increase the volume of air available for braking
  3. To reset the governor to its default cut-in pressure
  4. To remove condensed water and oil that can corrode components and freeze in lines Correct answer.

Explanation

Compressed air carries moisture and oil that condense in the reservoirs; draining removes this contamination. Left in place, water can corrode tanks and valves and freeze in cold weather, blocking lines. Draining does not reset the governor, release spring brakes, or add air capacity.

Question 9Air brake system components and function

In a dual air brake system, what is the function of the double check valve?

  1. It isolates a failed circuit so the other circuit loses no air at all
  2. It allows the higher-pressure circuit to supply air to a shared component while preventing backflow Correct answer.
  3. It ensures both circuits always deliver equal pressure to the brake chambers
  4. It drains water from the primary circuit automatically

Explanation

The double check valve is a two-inlet, one-outlet valve that passes whichever inlet has higher pressure and blocks flow back into the lower-pressure side. This lets either circuit supply shared items such as the trailer or parking brake system while isolating the other circuit. It does not equalize pressure between circuits or remove water.

Question 10Air brake system components and function

What is the function of a relay valve in an air brake system?

  1. It uses reservoir air to quickly apply and release the brake chambers in response to a control signal Correct answer.
  2. It limits the pressure applied to the front brakes during normal stops
  3. It holds the parking brakes applied when the engine is off
  4. It converts air pressure into an electrical signal for the ABS module

Explanation

A relay valve is mounted near the chambers it serves (often for the trailer or rear axle) and uses a small control signal to open a large reservoir-to-chamber passage. This shortens the time air takes to reach the chambers, speeding application and release. It does not convert pressure to electrical signals or hold parking brakes.

Question 11Air brake system components and function

What does the foot valve (treadle valve) do in an air brake system?

  1. It automatically adjusts the slack adjusters
  2. It meters air pressure to the brake chambers in proportion to how far the pedal is pushed Correct answer.
  3. It sets the parking brake when the pedal is fully released
  4. It supplies air to the compressor intake

Explanation

The foot valve is the driver's service brake control; depressing the treadle progressively opens passages that deliver increasing air pressure to the chambers, and releasing it exhausts that air. The amount of pressure depends on treadle travel. It does not set parking brakes or adjust slack.

Question 12Air brake system components and function

Which component prevents air from flowing back from a reservoir toward the compressor?

  1. An inversion valve
  2. A pressure regulator
  3. A quick release valve
  4. A one-way check valve Correct answer.

Explanation

One-way check valves are placed between the compressor, wet tank, and reservoirs to keep air from flowing backward when the compressor unloads or a line fails. This preserves stored pressure. Quick release valves speed exhaust at the chambers, and the inversion valve controls spring brakes during a failure.

Question 13Air brake system components and function

During a pre-trip inspection, a driver applies the service brakes and hears a steady hiss of escaping air that stops when the pedal is released. What does this most likely indicate?

  1. A leak in the service brake circuit that should be repaired before driving Correct answer.
  2. The air dryer purging moisture
  3. The compressor reaching its cut-out pressure
  4. Normal operation of the quick release valve exhausting air

Explanation

Air should not escape continuously while the brakes are held. A steady hiss during application that stops on release points to a leak in the service circuit, which must be fixed before the vehicle is driven. Normal exhaust occurs mainly on release, and compressor cut-out and dryer purge are brief, distinct events.

Question 14Air brake system components and function

What is the purpose of the alcohol evaporator or air dryer in cold weather?

  1. To raise the operating pressure of the system for better braking
  2. To warm the brake drums before use
  3. To lubricate the brake chambers internally
  4. To keep moisture from freezing in air lines and valves Correct answer.

Explanation

In freezing conditions, moisture in the air system can freeze in lines and valves and disable the brakes. An alcohol evaporator or air dryer removes or prevents that moisture from freezing. Neither device raises system pressure, warms drums, or lubricates chambers.

Question 15Air brake system components and function

What does the parking brake (yellow) valve do when pulled out?

  1. It exhausts air from the spring brake chambers, allowing the springs to apply the brakes Correct answer.
  2. It increases pressure to the service chambers for parking
  3. It closes the supply line to the trailer only
  4. It supplies air to the spring brake chambers to apply the brakes

Explanation

Pulling the yellow parking brake valve cuts off and exhausts air from the spring brake chambers, so the mechanical springs apply the brakes. Pushing it in restores air to release the springs. Applying brakes by adding air would be the opposite of how spring brakes work.

Question 16Air brake system components and function

A driver performs a static leak test: with the engine off and the brakes applied, pressure drops from 100 to 92 psi in one minute on a single vehicle. What is the best interpretation?

  1. The air dryer is purging too often
  2. The loss exceeds the typical allowable limit and the system needs inspection Correct answer.
  3. The governor is cutting in too early
  4. The loss is normal and no action is needed

Explanation

A common standard is that a single vehicle should lose no more than about 3 psi in one minute with the brakes applied (and about 2 psi with brakes released); a drop of 8 psi in a minute exceeds that. This indicates a leak that should be found and repaired. Governor and dryer behavior would not cause a steady pressure loss with the engine off.

Question 17Air brake system components and function

What is the function of the quick release valve?

  1. It quickly applies the spring brakes in an emergency
  2. It vents excess pressure from the compressor
  3. It speeds the release of air from brake chambers so brakes disengage faster Correct answer.
  4. It rapidly drains the supply reservoir

Explanation

A quick release valve is mounted near the brake chambers and exhausts chamber air locally when the control pressure is released, shortening release time compared with sending air all the way back to the foot valve. It does not apply spring brakes, drain reservoirs, or relieve compressor pressure.

Question 18Air brake system components and function

What is the purpose of the tractor protection valve?

  1. It automatically applies the trailer spring brakes during normal stops
  2. It closes the tractor-to-trailer air lines if trailer pressure is lost, protecting tractor air Correct answer.
  3. It regulates the compressor's output pressure
  4. It limits tractor speed when the trailer is disconnected

Explanation

The tractor protection valve senses trailer supply pressure and closes the coupling lines if that pressure drops, such as during a trailer breakaway, so the tractor retains enough air to stop. It also works with the trailer supply valve to charge and release the trailer. It does not govern speed or compressor output.

Question 19Air brake system components and function

The supply pressure to the trailer drops below about 45 psi while driving. What should the driver expect?

  1. The compressor will immediately raise pressure above 125 psi
  2. The trailer service brakes will release and the tractor will lose all braking
  3. The trailer spring brakes will apply and the tractor protection valve will close the lines Correct answer.
  4. The ABS light will turn off

Explanation

When trailer supply pressure falls to roughly 45 psi, the tractor protection valve closes the lines and the trailer spring brakes apply, acting as an emergency brake. The tractor retains its own air for stopping. The compressor cannot instantly over-pressurize the system, and the ABS light would not turn off from low supply pressure.

Question 20Air brake system components and function

Why are the primary and secondary circuits of a dual air brake system separated?

  1. To double the total air capacity available for braking
  2. So a failure in one circuit still leaves the other circuit able to stop the vehicle Correct answer.
  3. To let the driver choose which circuit to use
  4. To allow the compressor to fill both circuits at different pressures

Explanation

Dual circuits divide the system, typically front axle versus rear axle, so a leak or failure in one does not drain the other. The driver still has braking from the intact circuit. The design is about redundancy, not added capacity, and the driver does not select circuits.

Question 21Air brake system components and function

A driver finds that the spring brakes will not release after starting the engine and building pressure. Which condition best explains this?

  1. The slack adjusters are too tight
  2. The safety valve is stuck open
  3. Insufficient air pressure is reaching the spring brake chambers to compress the springs Correct answer.
  4. The service chambers are over-pressurized

Explanation

Spring brakes release only when enough air pressure reaches the spring chamber to compress the spring. If pressure is too low or the parking valve or lines are not delivering air, the springs stay applied. Over-pressurized service chambers, tight slack adjusters, or an open safety valve would not be the direct cause of springs failing to release.

Question 22Air brake system components and function

What is the role of the inversion valve in a system with spring brakes?

  1. It modulates spring brake application during a loss of primary or secondary pressure so braking is controlled rather than sudden Correct answer.
  2. It inverts the electrical signal to the ABS modulator
  3. It converts service brake pressure into parking brake pressure
  4. It reverses the direction of air flow in the supply line

Explanation

The inversion valve senses loss of service circuit pressure and applies the spring brakes in a modulated, gradual way instead of letting them slam on. This keeps the vehicle controllable during an emergency. It does not reverse supply flow, convert service to parking pressure, or process electrical ABS signals.

Question 23Air brake system components and function

How does an automatic slack adjuster differ from a manual slack adjuster?

  1. It measures air pressure and signals the governor
  2. It sets the parking brake automatically when the vehicle stops
  3. It adjusts brake clearance automatically as the brakes are applied and released during normal use Correct answer.
  4. It eliminates the need for a brake chamber

Explanation

An automatic slack adjuster takes up brake lining wear during normal brake applications, keeping pushrod stroke within limits without manual adjustment. It does not replace the brake chamber, set the parking brake, or sense pressure for the governor. Manual slack adjusters require periodic adjustment by a technician.

Question 24Air brake system components and function

A driver notices the air pressure gauge shows pressure building very slowly and the compressor runs almost constantly. Which component is the most likely cause?

  1. A worn compressor or a leak in the compressor discharge line Correct answer.
  2. A quick release valve stuck open at the chamber
  3. A governor stuck in the unloaded position
  4. An inversion valve stuck closed

Explanation

Slow pressure buildup with the compressor running constantly points to a compressor that cannot pump efficiently or a leak on the discharge side before the reservoirs. A governor stuck unloaded would stop the compressor from building pressure at all, and a stuck quick release or inversion valve affects brake application rather than charge rate.

Question 25Air brake system components and function

What is the function of the supply reservoir (wet tank) in a multi-tank air brake system?

  1. It supplies air directly to the brake chambers
  2. It receives air directly from the compressor and traps moisture and oil before air moves to the other reservoirs Correct answer.
  3. It stores air only for the parking brake circuit
  4. It regulates the pressure delivered to the foot valve

Explanation

The supply or wet tank is the first tank after the compressor and air dryer; it collects air and lets moisture and oil settle out before the air passes through check valves to the primary and secondary reservoirs. It does not feed the chambers directly or regulate foot valve pressure.

Question 26Air brake system components and function

In a typical air brake system, what does the compressor governor do?

  1. It limits the maximum pressure delivered to the brake chambers during a full brake application
  2. It controls the rate at which the spring brakes release after the parking valve is pushed in
  3. It unloads the compressor when reservoir pressure reaches the cut-out setting and lets it resume pumping at the cut-in setting Correct answer.
  4. It regulates the pressure sent to the trailer service line through the tractor protection valve

Explanation

The governor senses reservoir pressure and cycles the compressor between a cut-out pressure (about 120 to 125 psi) and a cut-in pressure (about 100 psi). It controls when the compressor pumps and when it idles unloaded, which keeps system pressure within the designed operating range.

Question 27Air brake system components and function

What is the purpose of the low pressure warning device in an air brake system?

  1. To automatically apply the spring brakes whenever the compressor cycles off
  2. To give the driver a visible or audible warning before air pressure drops too low to stop safely Correct answer.
  3. To warn the driver when the trailer has been disconnected from the tractor
  4. To release excess pressure from the supply reservoir when it exceeds the cut-out setting

Explanation

The low pressure warning must activate before pressure falls below about 60 psi, giving the driver notice that braking ability is being lost. On most systems a buzzer and warning light come on together, and the spring brakes will eventually apply on their own as pressure continues to drop.

Question 28Air brake system components and function

What does the application pressure gauge show the driver?

  1. The pressure stored in the supply (wet) reservoir at all times
  2. The pressure being delivered to the brake chambers during a brake application Correct answer.
  3. The pressure in the trailer emergency line
  4. The compressor's cut-in and cut-out settings

Explanation

An application pressure gauge is plumbed to show how much air is being sent to the brake chambers as the driver presses the treadle. It helps the driver judge how hard the brakes are being applied, especially on downgrades or slippery surfaces.

Question 29Air brake system components and function

What is the function of the brake chamber in an air brake system?

  1. It stores compressed air for use during emergency braking
  2. It removes moisture and oil from the air before it reaches the reservoirs
  3. It controls the sequence in which the tractor and trailer brakes apply
  4. It converts air pressure into the mechanical force that applies the foundation brake Correct answer.

Explanation

The brake chamber receives air from the valve system and uses a diaphragm and pushrod to convert that pressure into a mechanical push. The pushrod moves the slack adjuster and foundation brake, applying the brake shoes or pads.

Question 30Air brake system components and function

Why does a dual air brake system have a one-way check valve between the compressor and the supply reservoir?

  1. To limit the pressure the compressor can build in the system
  2. To drain moisture out of the supply line automatically
  3. To keep air from flowing back from the reservoirs to the compressor when it is unloaded Correct answer.
  4. To split the system into primary and secondary circuits

Explanation

When the governor unloads the compressor, the compressor stops building pressure, and a check valve prevents stored air from bleeding back through it. This keeps reservoir pressure intact during the unloaded cycle and protects the compressor from backflow.

Question 31Air brake system components and function

A driver hears a short, sharp burst of air from beneath the vehicle each time the compressor reaches its cut-out pressure. What is most likely happening?

  1. A brake chamber diaphragm has ruptured
  2. The low pressure warning is activating
  3. The safety valve is releasing pressure as designed Correct answer.
  4. The slack adjuster is out of adjustment

Explanation

A brief release at cut-out is normal: the safety valve or an unloader valve may vent a small amount of air to prevent overpressure. A continuous or repeated loud leak at other times would suggest a different fault, but a short burst at cut-out is expected behavior.

Question 32Air brake system components and function

What is the function of the spring brake chamber's caging bolt?

  1. To set the compressor cut-out pressure
  2. To adjust the clearance between the brake shoe and the drum
  3. To drain moisture from the spring brake reservoir
  4. To mechanically compress the power spring so the brake can be released for service or towing Correct answer.

Explanation

The caging bolt is a manual tool used to compress and hold the spring inside the spring brake chamber. It is used when there is no air pressure available to release the spring brakes, such as for towing or repairs. The bolt must be removed and stored properly before the vehicle returns to service.

Question 33Air brake system components and function

What does the spring brake priority valve (or inversion valve arrangement) do when pressure falls below a safe level?

  1. It bypasses the governor to allow higher system pressure
  2. It increases compressor output to restore pressure faster
  3. It ensures the spring brakes apply automatically before service brake pressure is fully lost Correct answer.
  4. It releases air from the trailer service line to prevent jackknifing

Explanation

The spring brake priority valve is designed to protect the parking/emergency brake function. When system pressure drops to a dangerous level, it allows the spring brakes to apply automatically rather than waiting for total pressure loss, so the vehicle stops safely.

Question 34Air brake system components and function

A driver applies the brakes and the pushrod stroke on one chamber is much longer than the others. What does this most likely indicate?

  1. That brake is out of adjustment or has a mechanical fault and needs inspection Correct answer.
  2. The governor cut-out setting is too high
  3. The low pressure warning circuit is faulty
  4. The air dryer is saturated with moisture

Explanation

Unequal or excessive pushrod stroke on one wheel points to a slack adjuster or foundation brake problem on that wheel. The driver should have it inspected and adjusted before relying on the brakes, since a long stroke reduces braking force and may indicate a worn or damaged component.

Question 35Air brake system components and function

What is the purpose of the trailer supply (emergency) line in a tractor-trailer air brake system?

  1. It charges the trailer reservoirs and signals the trailer to release its spring brakes Correct answer.
  2. It carries the service brake application signal to the trailer relay valve
  3. It provides electrical power to the trailer ABS
  4. It returns exhausted air from the trailer chambers to the tractor

Explanation

The supply line, often called the emergency line, delivers air to the trailer's reservoirs and keeps the trailer spring brakes released while pressure is present. If the line is broken or disconnected, pressure drops and the trailer spring brakes apply automatically.

Question 36Air brake system components and function

Why must the air compressor have an unloader mechanism?

  1. To release moisture from the compressor crankcase
  2. To let the compressor idle without building pressure once the reservoirs reach cut-out pressure Correct answer.
  3. To keep the compressor from exceeding engine RPM
  4. To allow the compressor to pump more air at low engine speed

Explanation

Once the reservoirs reach the governor's cut-out pressure, the unloader holds the compressor intake valves open so it pumps no air. This prevents overpressure and reduces wear and power draw while the system is at full pressure.

Question 37Air brake system components and function

What does the double check valve in a dual circuit system do when one circuit loses pressure?

  1. It opens the safety valve to vent the failed circuit
  2. It sends all remaining air to the trailer only
  3. It closes both circuits to prevent any further air loss
  4. It allows the intact circuit to continue supplying air while blocking backflow into the failed circuit Correct answer.

Explanation

A double check valve passes air from whichever source has higher pressure and blocks flow back toward the lower-pressure side. If one circuit fails, the other can still supply the affected components, preserving partial braking.

Question 38Air brake system components and function

A driver notices that the air pressure builds normally but the brakes feel weak and the application gauge reads low while the reservoir gauges read full. What is the most likely explanation?

  1. The air dryer is not removing moisture
  2. A fault in the foot valve or delivery side is limiting pressure to the chambers Correct answer.
  3. The compressor is failing to build pressure
  4. The governor cut-out setting is too low

Explanation

If reservoir pressure is normal but the application gauge stays low, the problem lies between the reservoirs and the brake chambers, such as a faulty foot valve or a delivery restriction. The compressor and governor are working because the reservoirs are full.

Question 39Air brake system components and function

How does the relay valve speed up brake response on a vehicle with long air lines?

  1. It bypasses the governor to raise system pressure during braking
  2. It increases the compressor's output when the brakes are applied
  3. It uses reservoir air locally at the valve to apply the brakes instead of waiting for air to travel from the foot valve Correct answer.
  4. It reduces the number of brake chambers that need to be filled

Explanation

The relay valve is mounted close to the brake chambers and takes a small control signal from the foot valve. It then opens a local passage from the reservoir to the chambers, so the large volume of air needed does not have to travel the full length of the vehicle.

Question 40Air brake system components and function

During a walk-around inspection, a driver finds that the drain valve on the supply reservoir releases a large amount of milky fluid. What does this most likely indicate?

  1. The safety valve is set too high
  2. Moisture and oil are accumulating because the air dryer or compressor is not functioning correctly Correct answer.
  3. The slack adjusters are over-adjusted
  4. The low pressure warning switch is stuck closed

Explanation

Milky fluid in a reservoir drain indicates excessive moisture and oil carryover, often from a saturated air dryer or a failing compressor. The system should be serviced, because moisture can freeze in cold weather and oil can damage valves and seals.

Question 41Air brake system inspection and leak tests

During a pre-trip inspection of a vehicle with a dual air brake system, the warning buzzer and low-air light must activate before the pressure in either service reservoir drops below what value?

  1. 45 psi
  2. 80 psi
  3. 60 psi Correct answer.
  4. 100 psi

Explanation

Federal inspection standards require the low-air warning to come on before either service reservoir falls below 60 psi. The warning alerts the driver while enough pressure remains to stop safely. Testing it during the pre-trip confirms the switch and warning devices work.

Question 42Air brake system inspection and leak tests

In a dual air brake system, which reservoir supplies the front (steering) axle brakes?

  1. The supply (wet) tank
  2. The secondary reservoir Correct answer.
  3. The rear service reservoir
  4. The primary reservoir

Explanation

In the standard dual system, the secondary circuit protects the front axle brakes so a rear circuit failure still leaves steering-axle stopping power. The primary circuit serves the rear axle brakes. The wet tank is only a supply/collection tank and does not directly feed a brake circuit.

Question 43Air brake system inspection and leak tests

While holding the brake pedal fully applied with the engine off and the system charged, the allowable pressure loss in a combination vehicle is:

  1. 2 psi in one minute
  2. 3 psi in one minute Correct answer.
  3. 4 psi in one minute
  4. 5 psi in one minute

Explanation

The static leak test for a combination vehicle allows no more than 3 psi loss in one minute with the pedal held down. A single vehicle has a tighter 2 psi per minute limit. Exceeding the limit means the system needs repair before it is placed in service.

Question 44Air brake system inspection and leak tests

A driver applies the service brakes with the engine running and the system fully charged, then holds the pedal. What does the applied leak test primarily check?

  1. Whether the low-pressure warning switch functions
  2. Whether the brake chambers and lines hold pressure under application Correct answer.
  3. Whether the governor cuts out at the correct pressure
  4. Whether the compressor builds pressure fast enough

Explanation

Holding the pedal applied with the engine running and a full system pressurizes the brake chambers and lines, exposing leaks that appear only when the brakes are applied. The test does not evaluate compressor build rate or governor cut-out. A pressure drop beyond the allowed limit signals a leak in the applied circuit.

Question 45Air brake system inspection and leak tests

With a fully charged system and the engine off, the pressure gauge drops from 120 psi to 90 psi in one minute with the brakes released. What does this most likely indicate?

  1. A properly functioning low-air warning switch
  2. An overcharged wet tank
  3. A leak in the system that must be repaired Correct answer.
  4. A normal drop caused by the governor cutting in

Explanation

A 30 psi loss in one minute with the brakes released far exceeds the allowable static leak rate and points to a leak in the air system. The governor does not cause pressure to fall while the engine is off. The vehicle should be taken out of service until the leak is found and fixed.

Question 46Air brake system inspection and leak tests

How is the compressor build-up rate checked during a pre-trip inspection?

  1. By watching the gauge while the brake pedal is held down
  2. By timing pressure rise from governor cut-in to cut-out with the engine at operating rpm Correct answer.
  3. By opening the drain cocks on each reservoir
  4. By measuring the pressure drop after the engine is shut off

Explanation

Build-up rate is measured by timing how long the compressor takes to raise pressure from cut-in to cut-out at normal operating rpm. A slow rise suggests a weak compressor, a slipping belt, or a leak. The other procedures test leaks or drain operation, not compressor output.

Question 47Air brake system inspection and leak tests

On a vehicle with a dual air brake system, the pressure gauge shows the primary circuit at 120 psi and the secondary circuit at 70 psi. What is the correct interpretation?

  1. The gauge is reading the wet tank, which is always lower
  2. The system is normal because one circuit is fully charged
  3. The secondary circuit has a leak or supply problem and needs inspection Correct answer.
  4. The primary circuit is overcharged and should be drained

Explanation

In a dual system, both circuits should build to normal operating pressure. A secondary circuit stuck at 70 psi while the primary is at 120 psi indicates a leak, a stuck check valve, or a supply restriction in that circuit. The vehicle should not be placed in service until the cause is found.

Question 48Air brake system inspection and leak tests

During the pre-trip inspection, the driver pumps the brake pedal to reduce air pressure. At what point should the spring brakes begin to apply automatically?

  1. When pressure falls between 20 and 45 psi Correct answer.
  2. When pressure falls to about 100 psi
  3. When pressure reaches zero
  4. When pressure falls to about 60 psi

Explanation

Spring brakes are held off by air pressure, so as pressure drops they begin to apply in the 20 to 45 psi range. This automatic application is a fail-safe feature. The driver should verify this during the pre-trip by continuing to reduce pressure and watching for the parking brake to set.

Question 49Air brake system inspection and leak tests

What is the purpose of the alcohol evaporator or air dryer in an air brake system?

  1. To increase compressor output pressure
  2. To lubricate the brake chambers
  3. To reduce the governor cut-out pressure
  4. To remove moisture and contaminants from the air supply Correct answer.

Explanation

Moisture in the air system can freeze in cold weather and corrode components, so an evaporator or dryer removes water vapor and contaminants. It does not change compressor output or cut-out pressure. Regular draining of reservoirs still supplements this device.

Question 50Air brake system inspection and leak tests

A driver drains the supply (wet) tank during the pre-trip inspection and finds a milky, oily emulsion. What does this most likely indicate?

  1. Normal condensation that requires no action
  2. A correctly functioning air dryer
  3. A failed compressor or inadequate drying that is passing oil and water into the system Correct answer.
  4. An overfilled brake chamber

Explanation

Milky, oily fluid in the wet tank indicates oil passing from the compressor combined with moisture, which can damage valves and cause brake failure. A properly functioning dryer and compressor should keep the tank relatively clean. The vehicle should be inspected and repaired before use.