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ND CDL Air Brakes study materials
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ND CDL Air Brakes practice questions

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Question 1Air Brake System Components and Function

On a vehicle with a dual air brake system, what is the primary purpose of having two separate air circuits?

  1. To allow the compressor to run at two different speeds
  2. To supply air only to the trailer while the tractor uses a separate supply
  3. To increase the total reservoir pressure above 150 psi
  4. To provide a backup circuit so partial braking remains if one circuit fails Correct answer.

Explanation

A dual air brake system splits the service brakes into two independent circuits so that a failure such as a leak or broken line in one circuit still leaves the other circuit able to apply some braking. This redundancy is the defining safety feature of dual systems and is why a total loss of braking is far less likely.

Question 2Air Brake System Components and Function

What is the function of the air compressor in a CDL air brake system?

  1. To store compressed air for later use
  2. To pump air into the storage reservoirs to build pressure Correct answer.
  3. To release air from the spring brakes during normal operation
  4. To reduce air pressure before it reaches the brake chambers

Explanation

The compressor is engine-driven and pumps air into the reservoirs, building the pressure that the rest of the system uses. Storage is the reservoir's job, so the compressor's role is generation rather than storage.

Question 3Air Brake System Components and Function

The governor on an air brake system controls what?

  1. The slack adjuster travel limit
  2. The rate at which the brake chambers release air
  3. The amount of air delivered to the trailer
  4. The compressor's cut-in and cut-out pressure points Correct answer.

Explanation

The governor senses reservoir pressure and engages or disengages the compressor at set cut-in and cut-out pressures, typically around 100 psi cut-in and 125 psi cut-out. This keeps system pressure within its designed operating range.

Question 4Air Brake System Components and Function

A driver notices the air pressure gauge climbing to about 130 psi and the compressor never seems to stop. Which component is the most likely cause?

  1. A failed governor that is not signaling cut-out Correct answer.
  2. A clogged alcohol evaporator
  3. A stuck relay valve in the open position
  4. A leaking brake chamber diaphragm

Explanation

The governor is what tells the compressor to stop pumping at the cut-out pressure. If pressure keeps rising past the normal cut-out point, the governor is the prime suspect because it is failing to unload the compressor.

Question 5Air Brake System Components and Function

What is the purpose of the supply reservoir (wet tank) in an air brake system?

  1. To hold air for the parking brakes only
  2. To collect moisture and contaminants before air reaches the dry reservoirs Correct answer.
  3. To regulate the compressor cut-out pressure
  4. To provide the primary service brake application pressure directly to the chambers

Explanation

Air from the compressor first enters the supply or wet tank, where moisture and oil tend to condense and settle so they can be drained. From there, air passes through the air dryer and into the dry reservoirs that feed the brake circuits.

Question 6Air Brake System Components and Function

The foot valve (treadle valve) in an air brake system serves which function?

  1. It sets the parking brake when the pedal is released
  2. It automatically drains moisture from the reservoirs
  3. It balances air pressure between the tractor and trailer
  4. It applies air pressure to the brake chambers in proportion to how far the pedal is pushed Correct answer.

Explanation

The foot valve is the driver's service brake control; pushing the treadle meters air pressure to the brake chambers, with more pedal travel delivering more pressure. It is the primary input that modulates braking force.

Question 7Air Brake System Components and Function

What is the role of the relay valve in a tractor-trailer air brake system?

  1. It prevents the trailer from being charged until the parking brake is released
  2. It reduces reservoir pressure to a safe level
  3. It converts electrical signals into air pressure
  4. It speeds up brake application and release at the rear by supplying air from a nearby reservoir Correct answer.

Explanation

A relay valve uses a small control signal to open a large local air supply, so rear brakes apply and release faster than if air had to travel all the way from the foot valve. This shortens lag time on long wheelbases and trailer combinations.

Question 8Air Brake System Components and Function

In a spring brake (emergency/parking) chamber, what actually applies the brake when air pressure is released?

  1. A hydraulic piston
  2. A counterweight lever
  3. A heavy coil spring Correct answer.
  4. A vacuum diaphragm

Explanation

Spring brakes use a large compressed coil spring that pushes the pushrod out and applies the brake whenever air pressure is removed. Air pressure compresses the spring to release the brake, so losing air automatically applies it.

Question 9Air Brake System Components and Function

Why do spring brakes apply automatically when system air pressure drops below a safe level?

  1. Because the foot valve locks in the applied position
  2. Because the spring is held compressed by air, so losing air lets the spring apply the brake Correct answer.
  3. Because the governor opens a bypass to the atmosphere
  4. Because the compressor reverses direction at low pressure

Explanation

The spring is mechanically compressed and held back by air pressure during normal operation. When pressure falls below the spring's release threshold, the spring extends and applies the brake, providing a fail-safe emergency function.

Question 10Air Brake System Components and Function

The parking brake control valve (yellow diamond knob) is pulled out. What happens in a typical system?

  1. The service brakes are applied with full reservoir pressure
  2. Air is exhausted from the spring brake chambers, allowing the springs to apply the brakes Correct answer.
  3. The trailer supply line is charged to 120 psi
  4. Air is supplied to the spring brake chambers to release them

Explanation

Pulling the parking brake knob exhausts air from the spring brake chambers so the springs extend and apply the brakes. Pushing the knob in re-supplies air to compress the springs and release the brakes.

Question 11Air Brake System Components and Function

What is the purpose of the tractor protection valve?

  1. To regulate compressor cut-out pressure
  2. To synchronize tractor and trailer ABS
  3. To close the tractor's air supply to the trailer if pressure drops or the trailer breaks away Correct answer.
  4. To drain moisture from the trailer reservoir

Explanation

The tractor protection valve guards the tractor's air supply; if trailer pressure drops or the trailer separates, it closes off and prevents the tractor from losing all its air. It also works with the trailer supply valve to protect the combination.

Question 12Air Brake System Components and Function

The red trailer supply (emergency) knob pops out while you are driving. What is the correct interpretation?

  1. The trailer has normal air pressure and you can continue
  2. Trailer air pressure has dropped and the trailer spring brakes have likely applied Correct answer.
  3. The trailer ABS is functioning normally
  4. The tractor's service brakes have failed

Explanation

The red knob is designed to pop out automatically when trailer supply pressure falls too low, which applies the trailer spring brakes. This signals an air loss or breakaway condition that must be investigated before moving.

Question 13Air Brake System Components and Function

What does the low air pressure warning device do, and when must it activate?

  1. It automatically applies the parking brake at 90 psi
  2. It shuts off the compressor at 100 psi
  3. It releases air from the wet tank at 60 psi
  4. It sounds a buzzer or shows a light before pressure drops below a safe operating level Correct answer.

Explanation

The warning device gives an audible and/or visual alert before pressure falls into an unsafe range, typically by about 60 psi in most systems. This gives the driver time to stop safely before spring brakes apply from low pressure.

Question 14Air Brake System Components and Function

A vehicle's air pressure gauge shows the system building pressure very slowly and the low pressure warning stays on for a long time. Which check is most relevant to this symptom?

  1. Checking for a compressor or governor malfunction and system leaks Correct answer.
  2. Checking slack adjuster free play
  3. Checking the ABS wheel sensors
  4. Checking the trailer light wiring

Explanation

Slow pressure buildup points to the air supply side: a weak or slipping compressor, a governor problem, or leaks that bleed off what the compressor produces. Slack adjusters, lights, and ABS sensors do not affect how fast reservoirs charge.

Question 15Air Brake System Components and Function

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

  1. To increase reservoir pressure above governor cut-out
  2. To cool the compressor
  3. To lubricate the brake chambers
  4. To remove moisture and contaminants from the compressed air before it reaches the reservoirs Correct answer.

Explanation

The air dryer uses a desiccant to absorb moisture and filter contaminants so that water does not accumulate in the reservoirs and freeze in lines. It protects valves and brake components from corrosion and ice.

Question 16Air Brake System Components and Function

Why must reservoir drain cocks be opened periodically, and what should a driver expect?

  1. To release built-up moisture and check for excessive water or oil in the air supply Correct answer.
  2. To test the trailer spring brakes
  3. To increase reservoir pressure before a trip
  4. To reset the governor cut-in point

Explanation

Compressed air carries moisture and oil that condense in the reservoirs, and draining removes this contamination. A driver should see only a small amount of water; excessive water or oil indicates a problem needing service.

Question 17Air Brake System Components and Function

The safety valve on an air reservoir is designed to do what?

  1. Maintain a minimum pressure for the parking brake
  2. Release air if reservoir pressure exceeds a safe maximum Correct answer.
  3. Automatically drain water from the tank
  4. Balance pressure between the two service circuits

Explanation

The safety valve is a pressure-relief device that vents air if reservoir pressure rises above a set maximum, protecting the tank and components from over-pressurization. It is a backup safeguard, not a normal pressure regulator.

Question 18Air Brake System Components and Function

What does the application pressure gauge on some vehicles show, and how does it differ from the reservoir gauge?

  1. It shows the pressure in the wet tank only
  2. It shows the pressure being delivered to the brake chambers during application Correct answer.
  3. It shows compressor output temperature
  4. It shows trailer ABS status

Explanation

An application gauge displays how much air pressure is being sent to the brakes as the driver applies the pedal, while the reservoir gauge shows stored supply pressure. The two readings answer different questions about system condition.

Question 19Air Brake System Components and Function

A driver applies the brakes and hears a steady hiss that continues after the pedal is released. What is the most likely concern?

  1. The compressor is unloaded
  2. The governor cut-in point is too high
  3. Normal air dryer purge cycle
  4. An air leak in the service brake system Correct answer.

Explanation

A continuous hiss after releasing the brakes suggests air is escaping from a service brake component or line rather than a normal purge. Leaks reduce available pressure and can lead to spring brake application if not corrected.

Question 20Air Brake System Components and Function

What is the function of the quick release valve in an air brake system?

  1. It rapidly drains the wet tank
  2. It releases the trailer supply line during a breakaway
  3. It quickly applies the parking brake in an emergency
  4. It speeds up release of air from brake chambers so brakes disengage faster Correct answer.

Explanation

A quick release valve exhausts air close to the brake chambers when the pedal is released, so the brakes release faster than if air had to travel back through long lines. This improves release timing and reduces brake drag.

Question 21Air Brake System Components and Function

Which component converts the driver's air pressure signal into a larger volume of air at the rear brakes on a long-wheelbase tractor?

  1. The slack adjuster
  2. The governor
  3. The alcohol evaporator
  4. The relay valve Correct answer.

Explanation

The relay valve responds to a small control pressure from the foot valve and opens a large local reservoir supply to the rear chambers. This reduces lag and provides adequate volume for rear brake application.

Question 22Air Brake System Components and Function

What does a slack adjuster do in an air brake system?

  1. It regulates air pressure to the trailer
  2. It adjusts the compressor belt tension
  3. It converts the pushrod stroke into rotation of the brake camshaft and takes up lining wear Correct answer.
  4. It drains moisture from the reservoirs

Explanation

The slack adjuster is the lever that connects the brake chamber pushrod to the camshaft, and its adjustment compensates for lining and drum wear. Proper adjustment keeps pushrod stroke within legal limits so brakes apply fully.

Question 23Air Brake System Components and Function

During a pre-trip inspection, a driver finds that the tractor protection valve does not close when the trailer supply line is disconnected. Why is this a serious defect?

  1. The compressor would overheat
  2. The tractor could lose its air supply through the open trailer line Correct answer.
  3. The ABS would disable the service brakes
  4. The trailer brakes would apply too quickly

Explanation

The tractor protection valve must close when the trailer line is disconnected or pressure is lost, so the tractor's air supply is not exhausted through the open line. If it fails to close, the tractor can lose air and its spring brakes may apply unexpectedly.

Question 24Air Brake System Components and Function

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

  1. To lubricate the brake chambers
  2. To increase compressor output
  3. To reduce governor cut-out pressure
  4. To prevent moisture in the air lines from freezing in cold weather Correct answer.

Explanation

Alcohol evaporators introduce alcohol vapor into the air system to lower the freezing point of any moisture present, helping prevent ice blockage in valves and lines during cold weather. They supplement, not replace, proper air drying and draining.

Question 25Air Brake System Components and Function

A driver performs a static leak test: with the engine off, brakes released, and full pressure, the gauge drops noticeably over one minute. What does this indicate?

  1. The governor is working correctly
  2. An air leak that must be located and repaired before driving Correct answer.
  3. The air dryer is regenerating
  4. Normal pressure loss from the compressor

Explanation

With the system charged and the engine off, pressure should remain stable or drop only within a small allowable amount. A noticeable drop indicates a leak in the system that must be repaired before the vehicle is operated.

Question 26Air Brake System Components and Function

In a typical air brake system, what is the function of the compressor governor?

  1. It regulates how much air pressure reaches the brake chambers during a stop.
  2. It drains moisture from the supply reservoir each time the compressor cycles.
  3. It limits the maximum pressure the compressor can physically produce internally.
  4. It controls when the compressor loads and unloads to keep reservoir pressure within a set range. Correct answer.

Explanation

The governor senses reservoir pressure and signals the compressor's unloading mechanism to stop pumping at the cut-out pressure and resume at the cut-in pressure. This cycling keeps system pressure within the designed operating range. It does not meter air to the chambers or act as a safety device.

Question 27Air Brake System Components and Function

What is the purpose of the supply reservoir in an air brake system?

  1. It collects condensation so that liquid never reaches the air dryer.
  2. It provides the reference pressure used by the application gauge.
  3. It holds a reserve of compressed air that feeds the other circuits through a one-way check valve. Correct answer.
  4. It stores air that has already been metered to the brake chambers.

Explanation

The supply reservoir is the storage tank filled directly by the compressor, and it protects the rest of the system with a one-way check valve so air cannot flow back toward the compressor. It supplies the primary and secondary circuits. It is not a metering device or a moisture trap.

Question 28Air Brake System Components and Function

A one-way check valve is installed between the compressor and the supply reservoir. What does it accomplish?

  1. It prevents air from bleeding back out of the reservoir when the compressor is unloaded. Correct answer.
  2. It splits compressor output evenly between the primary and secondary circuits.
  3. It releases excess pressure back to the atmosphere when cut-out is exceeded.
  4. It warms the incoming air so moisture does not condense in the tank.

Explanation

When the compressor unloads, its output stops, and without a check valve stored air could leak back through the compressor. The check valve holds reservoir pressure until the compressor builds again. Pressure relief is the safety valve's job, and circuit splitting is done by a manifold or double check valve arrangement.

Question 29Air Brake System Components and Function

What does the double check valve in a dual-circuit air brake system do?

  1. It compares the two circuits and warns the driver when they differ by more than 10 psi.
  2. It allows the higher of two supply pressures to pass while blocking backflow into the lower-pressure circuit. Correct answer.
  3. It equalizes pressure between the two circuits at all times.
  4. It lets the driver select which circuit supplies the trailer.

Explanation

A double check valve has two inlets and one outlet; it shifts to whichever inlet has higher pressure and seals off the other. This lets a circuit keep functioning when its normal supply is lost. It does not equalize the circuits, warn the driver, or provide a manual selection.

Question 30Air Brake System Components and Function

In a dual-circuit (split) air brake system, why are the circuits separated?

  1. So the parking brake can be applied with air instead of springs.
  2. So the compressor can charge one circuit at a time to reduce load.
  3. So the trailer brakes can be released independently of the tractor.
  4. So a failure in one circuit still leaves a portion of the service brakes usable. Correct answer.

Explanation

Splitting the system into primary and secondary circuits means a leak or failure in one does not disable all service braking. The remaining circuit still provides braking, and the low pressure warning alerts the driver. Parking brakes rely on springs, and the compressor charges circuits together.

Question 31Air Brake System Components and Function

What is the function of the inversion (R-8) valve in a spring brake system?

  1. It returns exhausted air to the supply reservoir for reuse.
  2. It converts a decreasing air signal into an increasing delivery pressure to release spring brakes. Correct answer.
  3. It reverses the direction of trailer air lines at the gladhands.
  4. It inverts the polarity of the stop lamp switch signal.

Explanation

The inversion valve senses the control pressure from the parking brake valve; as that pressure is reduced, the valve delivers air to hold off the spring brakes, allowing modulated emergency braking. It is used in systems where spring brakes also serve as a parking and emergency brake. It does not deal with gladhand orientation or electrical polarity.

Question 32Air Brake System Components and Function

What is the purpose of the front brake limiting valve (where installed) on some older vehicles?

  1. It reduces front brake application pressure in slippery conditions to help prevent wheel lockup. Correct answer.
  2. It prevents front brakes from ever applying so steering is unaffected.
  3. It limits how far the brake pedal can travel.
  4. It balances front and rear brake chamber sizes.

Explanation

A limiting valve, often labeled 'normal' and 'slippery', reduces the air pressure delivered to the front chambers in the slippery setting to lessen the chance of front wheel lockup. It does not disable the front brakes entirely. Modern vehicles generally rely on automatic limiting or ABS instead.

Question 33Air Brake System Components and Function

On a vehicle with automatic slack adjusters, what does the adjuster do during normal brake operation?

  1. It measures and displays the pushrod stroke on the dash.
  2. It maintains the correct pushrod stroke as linings wear, without driver adjustment. Correct answer.
  3. It releases the spring brake when supply pressure is restored.
  4. It meters air into the chamber to smooth brake application.

Explanation

An automatic slack adjuster senses excess pushrod travel during a brake application and ratchets the adjuster to restore the correct stroke as linings wear. This keeps chamber stroke within legal limits without manual adjustment. It does not control spring brake release, meter air, or display stroke.

Question 34Air Brake System Components and Function

During a brake application, the pushrod stroke on one chamber is measured at the limit for a clamp-type chamber. What is the correct interpretation?

  1. The chamber has failed and will not apply at all.
  2. The brakes are within adjustment and no action is needed.
  3. The brakes may be out of adjustment and the vehicle should be placed out of service until corrected. Correct answer.
  4. The slack adjuster is over-adjusted and should be backed off.

Explanation

A stroke at or beyond the adjustment limit indicates the brakes may be out of adjustment and cannot be relied on for stopping. The vehicle should be tagged out of service and the condition corrected before driving. A stroke at the limit is not acceptable, and the chamber is still applying.

Question 35Air Brake System Components and Function

What does the brake chamber do when air pressure is delivered to it during a service brake application?

  1. It converts air pressure into mechanical force that moves the pushrod and applies the brake. Correct answer.
  2. It reduces the delivered pressure before it reaches the slack adjuster.
  3. It stores the delivered air until the next application.
  4. It vents the delivered air to atmosphere to prevent overpressure.

Explanation

A service brake chamber contains a diaphragm; delivered air pressure pushes the diaphragm and plate, moving the pushrod against the slack adjuster to apply the brake. It is an actuator, not a storage or regulating device. The pushrod force is then multiplied mechanically through the slack adjuster.

Question 36Air Brake System Components and Function

On a combination vehicle, what does the trailer spring brake chamber do when the trailer supply line is disconnected?

  1. It applies the trailer brakes by spring force because hold-off air is lost. Correct answer.
  2. It vents all trailer reservoir air to the atmosphere.
  3. It transfers trailer air to the tractor's secondary circuit.
  4. It holds the trailer brakes released so the trailer can be moved.

Explanation

The trailer spring brakes are held off by air from the trailer supply line; when that line is disconnected or loses pressure, the springs apply the brakes. This is why the trailer cannot be moved with the supply line disconnected. Reservoir air is not dumped and air is not shared with the tractor.

Question 37Air Brake System Components and Function

A driver with a dual-circuit system sees the pressure in one circuit falling while the other holds steady. What does the low pressure warning system do in this situation?

  1. It activates when pressure in either circuit drops below the warning threshold, typically about 60 psi. Correct answer.
  2. It remains silent because one circuit is still at normal pressure.
  3. It activates only if the trailer supply line is disconnected.
  4. It activates only when both circuits drop below the threshold at the same time.

Explanation

The warning device is designed to alert the driver when pressure in either service circuit falls below the warning threshold, commonly around 60 psi. A single-circuit loss still reduces braking capability and must be brought to the driver's attention. It does not wait for both circuits to fail.

Question 38Air Brake System Components and Function

What is the purpose of the service line (blue) gladhand on a tractor-trailer combination?

  1. It supplies air to release the trailer spring brakes.
  2. It returns exhausted air from the trailer to the tractor.
  3. It carries the driver's service brake application signal to the trailer. Correct answer.
  4. It carries electrical signals for the trailer stop lamps.

Explanation

The blue service line delivers the pneumatic service brake signal from the tractor to the trailer's relay valve so the trailer brakes apply with the tractor. The red emergency (supply) line charges the trailer and holds off the spring brakes. The service line does not carry electrical signals or return exhaust air.

Question 39Air Brake System Components and Function

A tractor's red trailer supply line is charged and the blue service line is intact. What does the tractor protection valve do if the trailer breaks away and the supply line separates?

  1. It applies the tractor's spring brakes immediately.
  2. It closes to prevent the tractor's air from being lost through the open trailer line. Correct answer.
  3. It opens fully to supply the trailer with maximum air.
  4. It vents the tractor's primary reservoir to atmosphere.

Explanation

The tractor protection valve senses the loss of pressure in the trailer supply line and closes, keeping the tractor's air supply from escaping through the broken line. This preserves the tractor's ability to brake. It does not open fully, apply tractor spring brakes, or dump the primary reservoir.

Question 40Air Brake System Components and Function

What component limits the air pressure delivered to the trailer's service brakes when the trailer is lightly loaded, on trailers so equipped?

  1. The trailer spring brake chamber
  2. The trailer relay valve with its control signal Correct answer.
  3. The gladhand coupler seal
  4. The trailer air tank drain valve

Explanation

The trailer relay valve delivers air to the trailer brake chambers in proportion to the control signal it receives. In some systems, a load-sensing or proportioning arrangement modifies that control signal so lighter loads receive lower brake pressure. The spring brake chamber, drain valve, and coupler seal do not proportion pressure.

Question 41Air Brake System Components and Function

What is the function of the brake pedal free play in a treadle-valve air brake system?

  1. It prevents the foot valve from being damaged by overpressure.
  2. It allows the treadle to fully exhaust and return, ensuring the brakes release completely. Correct answer.
  3. It compensates for worn brake linings automatically.
  4. It sets the maximum application pressure the foot valve can deliver.

Explanation

Free play lets the treadle return fully so the foot valve can exhaust delivery air and release the brakes. Without it, residual pressure could keep the brakes dragging. Free play does not set maximum pressure, adjust for lining wear, or protect against overpressure.

Question 42Air Brake System Components and Function

A driver notices that the air pressure builds normally but the compressor will not restart after cut-out until pressure falls well below the normal cut-in point. Which component should be suspected?

  1. The slack adjuster
  2. The safety valve
  3. The quick release valve
  4. The governor Correct answer.

Explanation

The governor controls the cut-in and cut-out pressures; if it is faulty or out of adjustment, the compressor may not resume at the correct pressure. The safety valve vents overpressure, the quick release valve speeds brake release, and the slack adjuster is mechanical. None of these sets compressor cycling points.

Question 43Air Brake System Components and Function

What is the function of the alcohol evaporator in an air brake system?

  1. It supplies alcohol to the brake chambers as a lubricant.
  2. It adds alcohol vapor to the air system to help prevent moisture from freezing in valves and lines. Correct answer.
  3. It removes alcohol from the air before it reaches the reservoirs.
  4. It burns off excess alcohol to keep the compressor cool.

Explanation

An alcohol evaporator introduces alcohol vapor into the air system so that condensed moisture is less likely to freeze and block valves or lines in cold weather. It does not cool the compressor, remove alcohol, or lubricate brake chambers. Air dryers are the more common modern moisture-control device.

Question 44Air Brake System Components and Function

A driver performs a static leak test on a dual-circuit system: with the engine off and brakes released at full pressure, the gauge drops slowly but steadily. What does this most likely indicate?

  1. A normal condition caused by the compressor cooling down
  2. An overfilled supply reservoir
  3. A faulty application gauge that reads low
  4. A leak in the system that must be located and repaired Correct answer.

Explanation

With the engine off and brakes released, pressure should hold steady. A steady drop indicates an air leak somewhere in the system that should be traced and repaired. Compressor cooling does not cause a continuing drop, and the gauge reading is not the cause.

Question 45Air Brake System Components and Function

What does the safety valve on an air reservoir protect against?

  1. Moisture accumulation inside the reservoir
  2. Excessive pressure that could damage the reservoir or system components Correct answer.
  3. Loss of air pressure in the event of a compressor failure
  4. Backflow of air from the trailer to the tractor

Explanation

The safety valve is a spring-loaded relief device that opens if reservoir pressure exceeds a set limit, protecting the tank and components from overpressure. It does not maintain pressure during a compressor failure, remove moisture, or control backflow.

Question 46Air Brake System Components and Function

Why is the supply reservoir typically larger than the individual circuit reservoirs in a dual-circuit system?

  1. It must hold enough air to charge the circuits and support repeated brake applications. Correct answer.
  2. It is required to be larger by the gladhand design.
  3. It holds the air dryer desiccant cartridge.
  4. It stores air only for the trailer, which uses more air.

Explanation

The supply reservoir provides the bulk air storage that feeds the primary and secondary circuits through check valves, giving enough reserve for charging and repeated stops. Circuit reservoirs are smaller because they serve specific circuits. The supply reservoir does not house the dryer cartridge.

Question 47Air Brake System Components and Function

A driver applies the parking brake and hears air exhausting from the spring brake chambers. What is happening?

  1. The service brake chambers are being charged.
  2. Hold-off air is being released so the springs can apply the brakes. Correct answer.
  3. Supply air is being added to release the brakes.
  4. The reservoir safety valve is relieving overpressure.

Explanation

Applying the parking brake releases the hold-off air from the spring brake chambers, and that air exhausts as the springs apply the brakes. The sound of escaping air is normal during application. It is not supply air being added or a safety valve relieving pressure.

Question 48Air Brake System Components and Function

What is the purpose of the trailer spring brake priority valve in some trailer air systems?

  1. It limits the trailer's maximum speed by controlling brake pressure.
  2. It gives the trailer brakes priority over the tractor brakes during a stop.
  3. It directs supply air to release the trailer spring brakes before filling the trailer reservoirs. Correct answer.
  4. It prevents the trailer from being charged until the tractor is moving.

Explanation

A spring brake priority valve gives incoming supply air priority for releasing the trailer's spring parking brakes before filling the reservoirs. This describes the charging sequence, not priority between tractor and trailer braking during a stop.

Question 49Air Brake System Components and Function

During a pre-trip inspection, a driver finds that the air pressure builds to cut-out and then continues to rise slowly past the normal cut-out pressure until the safety valve releases. What is the most likely cause?

  1. A leaking service line gladhand
  2. A clogged drain cock
  3. A worn slack adjuster
  4. A governor or unloading mechanism fault allowing the compressor to keep pumping Correct answer.

Explanation

If the compressor does not unload at cut-out, pressure keeps rising until the safety valve relieves it, pointing to a governor or unloader fault. Gladhand leaks, slack adjuster wear, and a clogged drain cock do not cause pressure to exceed cut-out. The safety valve acting as the only limit is a warning sign of this fault.

Question 50Air Brake System Components and Function

What is the function of the relay valve's exhaust port in a tractor-trailer air brake system?

  1. It supplies air to the trailer supply line.
  2. It vents the tractor's primary reservoir when the parking brake is set.
  3. It releases air from the trailer reservoir to the atmosphere continuously.
  4. It releases air from the trailer brake chambers when the control signal is reduced. Correct answer.

Explanation

When the driver releases the brake pedal, the control signal to the relay valve drops and the valve exhausts air from the trailer brake chambers through its exhaust port so the brakes release promptly. It does not vent the reservoir continuously or supply the emergency line.