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LA CDL Air Brakes study materials
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LA CDL Air Brakes practice questions

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Question 1Air brake system components and function

In a typical air brake system, what is the primary job of the compressor?

  1. Drain moisture from the supply reservoirs
  2. Convert air pressure into mechanical force at the wheel
  3. Reduce air pressure before it reaches the brake chambers
  4. Build and maintain air pressure in the storage tanks Correct answer.

Explanation

The compressor is engine-driven and pumps air into the supply reservoirs to keep system pressure within the governor's operating range. It is the source of the air that every other component uses. Moisture removal and pressure reduction are handled by other parts.

Question 2Air brake system components and function

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

  1. It controls when the compressor loads and unloads to regulate reservoir pressure Correct answer.
  2. It limits how far the brake pedal can travel
  3. It vents air from the spring brake chambers during parking
  4. It balances air pressure between the front and rear service circuits

Explanation

The governor senses reservoir pressure and signals the compressor to stop pumping (unload) at the cut-out pressure and to resume (load) at the cut-out minus the cut-in differential. This keeps system pressure within a safe operating band. It does not meter pedal travel or balance circuits.

Question 3Air brake system components and function

A driver notices the air pressure gauge climbs to about 130 psi and then the compressor seems to stop building, dropping back to roughly 100 psi before building again. Which component is creating this cycling?

  1. The alcohol evaporator
  2. The safety valve
  3. The low-pressure warning switch
  4. The governor Correct answer.

Explanation

The governor cuts out the compressor at the upper set point and cuts it back in at a lower pressure, producing this normal cycle. The safety valve and warning switch do not control compressor loading. The observed pattern is exactly the governor's load/unload behavior.

Question 4Air brake system components and function

What is the purpose of the safety valve installed on the air reservoir system?

  1. It prevents air from flowing back from the tanks to the compressor
  2. It sounds an alarm in the cab whenever pressure drops below 60 psi
  3. It automatically drains water from the tank each time the compressor cycles
  4. It releases air if system pressure rises above a preset maximum Correct answer.

Explanation

The safety valve is a pressure-relief device that protects the system from over-pressurization if the governor fails. It opens at its preset limit and closes when pressure falls back. It is not a drain, check valve, or warning device.

Question 5Air brake system components and function

Which component prevents pressurized air in the reservoirs from bleeding back through the compressor when the compressor unloads?

  1. The inversion valve
  2. The relay valve
  3. The one-way check valve at the compressor discharge Correct answer.
  4. The quick-release valve

Explanation

A check valve on the compressor discharge line allows air to flow only toward the reservoirs, holding pressure when the compressor stops pumping. Without it, tank air could escape back through the unloaded compressor. The other listed valves serve brake application and release functions.

Question 6Air brake system components and function

On a dual-circuit air brake system, what is the main reason the supply is split into two separate circuits?

  1. So a failure in one circuit still leaves braking capability in the other Correct answer.
  2. To let the driver choose which axle brakes to use
  3. To allow the compressor to run at two different pressures
  4. To reduce the total volume of air the compressor must move

Explanation

Dual circuits isolate a leak or failure so that the remaining circuit can still provide braking and pressure. This redundancy is a core safety requirement. The split is not about compressor pressure levels, driver axle selection, or reducing air volume.

Question 7Air brake system components and function

What does the low-pressure warning device on an air brake vehicle do?

  1. Automatically applies the parking brakes at 90 psi
  2. Warns the driver when system pressure falls into an unsafe range Correct answer.
  3. Locks the brake pedal until pressure recovers
  4. Signals the governor to restart the compressor

Explanation

A buzzer and/or light activates when reservoir pressure drops below a safe threshold so the driver knows braking may be compromised. In many systems the spring brakes also apply automatically at a lower pressure, but the warning device itself only alerts. It does not control the governor or lock the pedal.

Question 8Air brake system components and function

Which gauge reading tells the driver the pressure available to the service brake system?

  1. The speedometer
  2. The primary (and secondary) air pressure gauge Correct answer.
  3. The oil pressure gauge
  4. The tachometer

Explanation

The air pressure gauge shows reservoir pressure available for braking. Drivers must monitor it because low readings mean reduced braking ability. Speed, engine RPM, and engine oil pressure are unrelated to air brake supply.

Question 9Air brake system components and function

What is the role of the foot valve (treadle valve) in an air brake system?

  1. It removes moisture from the air before it reaches the tanks
  2. It meters air pressure to the brake circuits in proportion to pedal force Correct answer.
  3. It releases air from the spring brake chambers when parking
  4. It stores compressed air for emergency use

Explanation

The foot valve is the driver's service brake control; pressing the treadle admits air at a pressure proportional to how hard the pedal is pushed. It does not store air, release parking brakes, or dry the air.

Question 10Air brake system components and function

A driver presses the brake pedal lightly and hears air flowing, but the vehicle barely slows. Which statement best explains how the foot valve should behave?

  1. It should deliver pressure proportional to pedal force, so a light press gives light braking Correct answer.
  2. It should send full reservoir pressure to the chambers on any pedal movement
  3. It should delay air delivery until the pedal is fully depressed
  4. It should send air only to the front circuit regardless of pedal force

Explanation

The treadle valve is designed to meter air in proportion to how far and how hard the pedal is applied, giving graduated braking. Full pressure on any touch, front-only delivery, or a delay until full depression would all be incorrect behavior.

Question 11Air brake system components and function

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

  1. It holds the parking brakes applied when air pressure is lost
  2. It equalizes pressure between the two supply reservoirs
  3. It uses a small control signal to quickly supply or exhaust a large volume of air at the brakes Correct answer.
  4. It converts electrical signals into air pressure at the pedal

Explanation

A relay valve is mounted near the brakes it serves and uses a low-volume control signal from the foot valve to open a large supply of air locally. This shortens response time and release time on long vehicles. It does not convert electrical signals, hold parking brakes, or equalize reservoirs.

Question 12Air brake system components and function

Why are relay valves commonly used on the rear service brakes of long combination vehicles?

  1. They shorten the time needed to apply and release the rear brakes Correct answer.
  2. They increase the maximum pressure the compressor can produce
  3. They allow the rear brakes to work without any control signal
  4. They eliminate the need for slack adjusters

Explanation

Placing a relay valve near the rear brakes lets a small control signal trigger a large local air supply, reducing lag in apply and release. This improves stopping distance and brake balance. It does not change compressor output, replace slack adjusters, or work without a signal.

Question 13Air brake system components and function

What is the purpose of a quick-release valve?

  1. It quickly fills the reservoirs after start-up
  2. It rapidly exhausts air from the brake chambers when the brake is released Correct answer.
  3. It releases the spring brakes automatically at low pressure
  4. It dumps air from the safety valve when pressure is high

Explanation

A quick-release valve is located near the brake chambers and vents chamber air to atmosphere as soon as the control pressure drops. This speeds brake release and reduces drag. It does not fill reservoirs, release spring brakes, or act as a safety valve.

Question 14Air brake system components and function

What is the function of the slack adjuster?

  1. It releases air from the chamber when the pedal is released
  2. It maintains a fixed air pressure in the reservoir
  3. It converts the linear pushrod stroke of the brake chamber into rotation of the brake camshaft Correct answer.
  4. It regulates compressor cut-in and cut-out pressure

Explanation

The slack adjuster is a lever on the brake camshaft that turns chamber pushrod travel into camshaft rotation, applying the brakes. Automatic slack adjusters also compensate for lining wear. They do not store air, exhaust air, or govern compressor pressure.

Question 15Air brake system components and function

An automatic slack adjuster is installed, but a driver finds excessive pushrod stroke on one chamber. What is the most appropriate response?

  1. Crank the adjuster by hand and continue the trip without inspection
  2. Ignore it because automatic adjusters never need attention
  3. Increase reservoir pressure by adjusting the governor
  4. Have the brake system inspected and repaired by a qualified mechanic before driving Correct answer.

Explanation

Excessive pushrod stroke means the brake may not apply fully, and with an automatic slack adjuster it often signals a mechanical problem rather than normal wear. The correct action is inspection and repair by a qualified mechanic. Manual cranking without diagnosis or raising pressure is unsafe and does not fix the cause.

Question 16Air brake system components and function

What is the primary function of the spring brake (parking brake) chamber?

  1. It regulates pressure between the front and rear circuits
  2. It provides additional service braking while the pedal is pressed
  3. It stores air for the service brake circuit
  4. It applies the brakes mechanically when air pressure is released from the spring cavity Correct answer.

Explanation

A heavy spring applies the brakes when air is released from the spring cavity, so the brakes set if pressure is lost. Air pressure compresses the spring to release the brake. The spring brake is separate from the service chamber and does not store air or balance circuits.

Question 17Air brake system components and function

A truck loses all air pressure while parked with the engine off. What should happen to the spring brakes?

  1. They release and allow the vehicle to roll
  2. They apply automatically and hold the vehicle Correct answer.
  3. They remain in whatever position they were last set
  4. They cycle on and off as pressure bleeds down

Explanation

Spring brakes are held off by air pressure; when pressure is lost, the springs extend and apply the brakes. This fail-safe design is why a vehicle with no air cannot be driven normally. They do not release, freeze in place, or cycle.

Question 18Air brake system components and function

What does the parking brake control valve do when pulled to the applied position?

  1. It closes the supply line to the compressor
  2. It opens the safety valve to vent the reservoirs
  3. It supplies air to the spring brake chambers to apply them
  4. It exhausts air from the spring brake chambers so the springs apply Correct answer.

Explanation

The parking valve releases air from the spring cavities, letting the springs push the brakes on. Pushing the valve in re-supplies air to compress the springs and release the brakes. It does not apply brakes by adding air, close the compressor line, or trip the safety valve.

Question 19Air brake system components and function

What is the purpose of the inversion valve in a spring brake system?

  1. It changes the polarity of the low-pressure warning circuit
  2. It provides a modulated spring brake application when service air pressure is lost Correct answer.
  3. It inverts the gauge reading so low pressure shows as high
  4. It reverses the direction of air flow through the compressor

Explanation

The inversion valve allows a controlled, modulated application of the spring brakes during an emergency rather than a sudden full lock-up. This gives the driver some braking control after a service system failure. It has nothing to do with compressor flow or gauge or electrical polarity.

Question 20Air brake system components and function

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

  1. It limits the maximum pressure in the system
  2. It cools the compressor by circulating air around it
  3. It stores air for the parking brake circuit
  4. It removes moisture and contaminants from the air before it reaches the reservoirs Correct answer.

Explanation

The air dryer uses a desiccant and filter to remove water vapor and oil or dirt from compressor discharge air, helping prevent corrosion and freeze-ups. It does not cool the compressor, store air, or limit system pressure.

Question 21Air brake system components and function

During a pre-trip inspection, a driver drains the reservoirs and notices a large amount of water and milky fluid. What does this most likely indicate?

  1. The air dryer is not removing moisture properly and needs service Correct answer.
  2. The safety valve is set too low
  3. The compressor is producing too much pressure
  4. The slack adjusters are out of adjustment

Explanation

Water and milky fluid in the reservoirs point to moisture carryover, often from a saturated or failed air dryer or a missing alcohol evaporator. The system should be serviced before driving. Excess pressure, safety valve setting, and slack adjustment do not produce water in the tanks.

Question 22Air brake system components and function

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

  1. It lubricates the compressor with an alcohol-based oil
  2. It cleans the alcohol from the air before it reaches the valves
  3. It adds alcohol vapor to the air to help prevent moisture from freezing in the lines Correct answer.
  4. It burns alcohol as fuel to heat the cab

Explanation

An alcohol evaporator introduces alcohol vapor into the air system so any moisture present is less likely to freeze and block lines or valves in cold weather. It is not a fuel burner, an air cleaner, or a compressor lubricant.

Question 23Air brake system components and function

What is the purpose of the drain cocks on the air reservoirs?

  1. To release air to the brakes during an emergency
  2. To adjust system pressure to the desired level
  3. To connect a test gauge to the compressor
  4. To let the driver release accumulated water and oil from the tanks Correct answer.

Explanation

Drain cocks at the bottom of each reservoir allow accumulated moisture and oil to be blown out, usually daily. Keeping tanks dry protects valves from corrosion and freezing. They are not emergency brake controls, pressure regulators, or gauge ports.

Question 24Air brake system components and function

A driver builds full pressure, shuts off the engine, and watches the gauge for one minute with the brakes released. The pressure drops noticeably. What does this indicate?

  1. The governor is working correctly to hold pressure
  2. An air leak that should be located and repaired before driving Correct answer.
  3. The safety valve is protecting the system as designed
  4. Normal behavior because compressors always leak down

Explanation

With the engine off and brakes released, system pressure should hold steady except for a very small allowable loss. A noticeable drop signals a leak in the system that must be found and fixed. Compressors do not cause a leak-down when the engine is off, and the governor or safety valve would not produce this symptom.

Question 25Air brake system components and function

Which component senses reservoir pressure and signals the compressor to stop pumping at the cut-out point?

  1. The governor Correct answer.
  2. The low-pressure indicator
  3. The relay valve
  4. The foot valve

Explanation

The governor is the pressure-sensing control that unloads the compressor at cut-out and reloads it at cut-in. Relay and foot valves control brake application air, and the indicator only warns the driver.

Question 26Air brake system components and function

On a CDL air brake pre-trip inspection, what does the compressor governor do when reservoir pressure reaches the cut-out setting?

  1. It engages the spring brakes until pressure drops below cut-in
  2. It opens the safety valve to vent excess pressure to the atmosphere
  3. It unloads the compressor so it stops building pressure into the reservoirs Correct answer.
  4. It closes the one-way check valve between the compressor and the wet tank

Explanation

The governor senses reservoir pressure and controls the compressor's unloading mechanism. At cut-out it lets the compressor idle without pumping air; at cut-in it restores pumping. This cycling keeps pressure within the designed operating range.

Question 27Air brake system components and function

Which component prevents air from bleeding back from the reservoirs toward the compressor when the compressor stops pumping?

  1. The pressure relief cock
  2. The low-pressure indicator
  3. The safety valve
  4. The one-way check valve Correct answer.

Explanation

A one-way check valve is installed in the compressor discharge line so air can flow into the reservoirs but cannot flow back. This holds pressure in the system while the compressor is unloaded or stopped. Without it, reservoir pressure would leak down through the compressor.

Question 28Air brake system components and function

What is the purpose of the safety valve installed on an air brake reservoir?

  1. To balance pressure between the primary and secondary circuits
  2. To release air automatically when pressure falls below the cut-in point
  3. To release air automatically if system pressure rises above a safe maximum Correct answer.
  4. To drain moisture from the bottom of the tank during daily inspections

Explanation

The safety valve is a spring-loaded relief device set slightly above normal governor cut-out. If the governor or compressor malfunctions and pressure climbs too high, the valve vents air to protect tanks and lines from rupture. It does not operate during normal pressure cycling.

Question 29Air brake system components and function

A driver applies the parking brake control valve and then releases it. What happens inside the spring brake chambers during this sequence?

  1. The spring is allowed to expand to apply the brakes, then air pressure compresses it to release them Correct answer.
  2. The spring is removed from the circuit and service air alone holds the vehicle
  3. The spring is compressed to release the brakes, then allowed to expand to apply them
  4. Air pressure is trapped in the spring cavity to hold the brakes applied, then vented to release them

Explanation

Pulling the parking control valve exhausts air from the spring cavity, so the large spring expands and applies the brakes. Pushing the valve in supplies air that compresses the spring and releases the brakes. This is why the brakes stay applied whenever air pressure is lost.

Question 30Air brake system components and function

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

  1. It receives air directly from the compressor and feeds the other circuits through check valves Correct answer.
  2. It stores air for the front service brakes only
  3. It holds air only for the trailer emergency line
  4. It supplies air only to the spring brake chambers

Explanation

The supply reservoir is the first tank downstream of the compressor and collects moisture and oil before air passes on. Check valves let it feed the primary and secondary circuits while preventing backflow between them. This arrangement helps isolate a failed circuit from the rest of the system.

Question 31Air brake system components and function

A driver notices that the service brakes on one axle apply late and release slowly compared with the other axles. Which component is most likely restricting flow in that branch?

  1. The air dryer purge valve
  2. The compressor discharge line
  3. A stuck or clogged valve in that axle's delivery line Correct answer.
  4. The governor

Explanation

Delayed apply and slow release on a single axle point to restricted flow in that branch, such as a clogged line or a sticking relay or quick-release valve. The rest of the system works normally, which isolates the fault to that circuit. Governors and compressors affect the whole system, not one axle.

Question 32Air brake system components and function

What does the tractor protection valve do when the trailer air supply is lost or the trailer breaks away?

  1. It disables the tractor's own service brakes
  2. It dumps all tractor reservoir pressure to the atmosphere
  3. It opens fully to send maximum air to the trailer brakes
  4. It closes to keep the tractor's air from escaping through the trailer lines Correct answer.

Explanation

The tractor protection valve senses trailer supply pressure and closes when it drops too low or when the trailer separates. This traps the tractor's air so the power unit can still brake and the trailer line does not bleed the system dry. It also signals the trailer to apply its spring brakes.

Question 33Air brake system components and function

During a pre-trip test, a driver fanning the brake pedal watches the gauge fall and then stabilize at a pressure below cut-in without the compressor restarting. What does this most likely indicate?

  1. The safety valve is opening too early
  2. The governor is stuck in the unloaded position or set incorrectly Correct answer.
  3. The spring brakes are out of adjustment
  4. The trailer emergency line is disconnected

Explanation

The governor should return the compressor to the loaded state at cut-in so pressure rebuilds. If pressure stays low and the compressor does not resume pumping, the governor is not signaling it to load. This is a governor or compressor control fault rather than a brake adjustment issue.

Question 34Air brake system components and function

What is the function of the brake chamber diaphragm in a service brake assembly?

  1. It stores reserve air for repeated brake applications
  2. It releases air to the atmosphere when the pedal is released
  3. It automatically adjusts pushrod stroke as linings wear
  4. It converts air pressure into mechanical force on the pushrod Correct answer.

Explanation

Air pressure entering the chamber pushes against the diaphragm, which flexes and moves the pushrod to apply the brake. The diaphragm separates the pressure side from the pushrod side and provides a flexible seal. Its movement is what transfers pneumatic force into mechanical braking force.

Question 35Air brake system components and function

A driver hears a continuous hiss from the engine area whenever the compressor is running, and reservoir pressure builds very slowly. Which condition best explains this?

  1. An incorrectly adjusted slack adjuster
  2. A leaking compressor discharge line or fitting on the high-pressure side Correct answer.
  3. A clogged drain cock on the wet tank
  4. A worn service brake diaphragm that leaks only during application

Explanation

A leak on the compressor discharge side lets pressurized air escape continuously while the compressor pumps, so pressure rises slowly. The hiss occurring only during compressor operation is the key clue. Leaks downstream of the reservoirs behave differently because they drain stored air even when the compressor is unloaded.

Question 36Air brake system components and function

What is the purpose of the front brake limiting valve found on some older air brake vehicles?

  1. It automatically applies the front brakes if the rear circuit fails
  2. It increases front brake pressure whenever the pedal is fully applied
  3. It isolates the front brakes from the warning system
  4. It reduces air pressure to the front brakes in the normal position to help prevent wheel lockup Correct answer.

Explanation

The limiting valve, often labeled normal and slippery, cuts front brake application pressure in its normal setting to reduce the chance of front wheel lockup on some older vehicles. Drivers select the slippery position on wet or icy roads. Modern vehicles generally use proportioning or ABS instead.

Question 37Air brake system components and function

A driver must check that the spring brakes apply automatically when air pressure drops. Which procedure correctly tests this on a vehicle with the engine running?

  1. Build full pressure, shut off the engine, and listen for the compressor to cycle
  2. Drain the wet tank only and check that the front brakes hold
  3. Release the parking brake and pump the pedal once to confirm the low-pressure light comes on
  4. Build full pressure, then repeatedly fan the brake pedal and watch for the spring brakes to set as pressure falls Correct answer.

Explanation

Fanning the service brakes with the engine running and the parking brake released lowers reservoir pressure without the compressor keeping up. As pressure falls toward the warning range, the spring brakes should apply automatically and the low-pressure warning should activate. This verifies the protection feature works before a trip.

Question 38Air brake system components and function

Why does a dual-circuit air brake system use separate check valves between the supply reservoir and the primary and secondary reservoirs?

  1. To allow each circuit to draw from the supply tank while preventing one circuit's loss from draining the other Correct answer.
  2. To route trailer air directly to the spring brake chambers
  3. To equalize pressure so both circuits always read exactly the same on the gauge
  4. To vent excess pressure from the secondary circuit to the atmosphere

Explanation

Check valves let air flow from the supply reservoir into each circuit but block reverse flow between circuits. If one circuit develops a major leak, the other can still receive air and keep working. This isolation is central to the safety of a dual-circuit system.

Question 39Air brake system components and function

A combination vehicle loses its trailer supply line. Which sequence of events should occur?

  1. The tractor protection valve closes, the trailer spring brakes apply, and the tractor keeps its air Correct answer.
  2. The tractor spring brakes apply while the trailer brakes stay released
  3. The trailer service brakes release and the tractor loses all reservoir pressure
  4. The trailer supply line vents the wet tank to atmosphere through the drain cock

Explanation

Loss of trailer supply pressure causes the tractor protection valve to close and stop feeding the trailer line. The trailer then loses emergency pressure and its spring brakes apply automatically. The tractor retains its own air so the driver can still brake and stop safely.

Question 40Air brake system components and function

A vehicle's compressor keeps running but pressure never reaches cut-out, and no leak can be heard anywhere on the vehicle. Which explanation is most consistent with these findings?

  1. The compressor is worn and cannot build pressure faster than the system's normal bleed-off Correct answer.
  2. The governor is stuck in the loaded position
  3. The safety valve is stuck closed
  4. The low-pressure warning switch is faulty

Explanation

If the compressor runs constantly without reaching cut-out and no audible leak exists, the compressor may be too worn to overcome normal system bleed-off. A stuck governor would also keep it running, but the question rules out control faults by stating pressure never builds. Worn compressor output is the remaining explanation.

Question 41Inspecting the air brake system (pre-trip and in-cab checks)

During a pre-trip inspection, what should happen to the air pressure gauge reading when you release the parking brake after the system is fully charged and the engine is off?

  1. It climbs because the compressor recharges the tanks.
  2. It drops to zero until the engine is restarted.
  3. It stays exactly the same because the parking brake is spring applied.
  4. It drops slightly as air fills the brake chambers, then holds steady. Correct answer.

Explanation

With a fully charged system and the engine off, releasing the parking brake admits air into the spring brake chambers, so the gauge should dip slightly and then hold. A steady reading with no dip, or a continued fall, indicates a leak or a delivery problem that must be found before driving.

Question 42Inspecting the air brake system (pre-trip and in-cab checks)

What does the low air pressure warning device on a CDL vehicle have to do?

  1. Activate only after the spring brakes have fully applied.
  2. Sound continuously whenever the engine is running.
  3. Sound a buzzer or show a light before pressure falls below 60 psi. Correct answer.
  4. Trigger at exactly 100 psi during normal operation.

Explanation

Federal standards require a warning signal that activates before the air pressure drops below 60 psi, giving the driver time to stop safely. It should be checked during the in-cab portion of the pre-trip by fanning the brakes down and watching for the warning.

Question 43Inspecting the air brake system (pre-trip and in-cab checks)

When you perform the static leak test with the engine off and the system fully charged, what is the acceptable pressure loss in one minute with the brakes released?

  1. No more than 20 psi.
  2. No more than 3 psi. Correct answer.
  3. No more than 10 psi.
  4. Any loss is acceptable if the buzzer stays off.

Explanation

With a fully charged system, the engine off, and the brakes released, pressure should not drop more than 3 psi in one minute. A larger drop means a leak that must be repaired before the vehicle is placed in service.

Question 44Inspecting the air brake system (pre-trip and in-cab checks)

How much air pressure loss is permitted in one minute during the applied leak test with the brakes fully applied and the engine off?

  1. No more than 8 psi.
  2. No more than 2 psi.
  3. No more than 15 psi.
  4. No more than 4 psi. Correct answer.

Explanation

With the service brakes fully applied and the engine off, the system may lose no more than 4 psi in one minute. This test checks the applied side of the system, including the chambers and delivery lines, which the released test does not stress.

Question 45Inspecting the air brake system (pre-trip and in-cab checks)

During the in-cab check, you fan the brake pedal and watch the gauge. At what pressure should the spring brakes begin to apply on a typical CDL vehicle?

  1. Between 60 and 80 psi.
  2. Only after the gauge reaches zero.
  3. Between 20 and 45 psi. Correct answer.
  4. At exactly 100 psi.

Explanation

As pressure falls, the spring brakes are designed to apply automatically somewhere in the 20 to 45 psi range, well below the low-pressure warning point. This automatic application is what makes a total air loss survivable rather than catastrophic.

Question 46Inspecting the air brake system (pre-trip and in-cab checks)

You are checking the air compressor's ability to build pressure. With the engine at operating rpm and the system at a normal cut-in point, what is the usual requirement for build-up time?

  1. Pressure should rise from 85 to 100 psi within about 5 minutes.
  2. Pressure should rise only while the vehicle is moving.
  3. Pressure should rise from 85 to 100 psi within about 45 seconds. Correct answer.
  4. Pressure should stay flat until the governor cuts in.

Explanation

A healthy compressor should raise pressure from roughly 85 to 100 psi in about 45 seconds at operating rpm. A much slower build indicates a weak compressor, a slipping belt, or a leak that needs attention.

Question 47Inspecting the air brake system (pre-trip and in-cab checks)

At what pressure should the governor typically cut out the compressor on an air brake system?

  1. Around 60 psi.
  2. Around 125 psi. Correct answer.
  3. Around 85 psi.
  4. Around 150 psi.

Explanation

The governor usually cuts the compressor out near 125 psi and cuts it back in around 85 to 100 psi, keeping the system in a safe operating band. Gauges should be watched during the pre-trip to confirm the compressor cycles in this range.

Question 48Inspecting the air brake system (pre-trip and in-cab checks)

During the pre-trip inspection, you drain the air tanks. What should you look for as each tank drains?

  1. A small amount of moisture and no oil or heavy sludge. Correct answer.
  2. Compressor coolant mixed with the air.
  3. A steady stream of oil.
  4. Nothing at all should come out.

Explanation

Draining the tanks removes accumulated moisture and confirms the drain valves work. A little water is normal, but oil, coolant, or heavy sludge means the compressor or cooling system needs service.

Question 49Inspecting the air brake system (pre-trip and in-cab checks)

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

  1. To remove moisture that could freeze in lines and valves. Correct answer.
  2. To lubricate the brake chambers.
  3. To increase the maximum system pressure.
  4. To reduce the noise of the compressor.

Explanation

Moisture in the air system can freeze in cold weather and block valves or lines, so an evaporator or dryer removes it. During the pre-trip you should confirm the device is present and, where applicable, has an adequate alcohol supply.

Question 50Inspecting the air brake system (pre-trip and in-cab checks)

You are inspecting the slack adjusters on a vehicle with automatic slack adjusters. Which finding indicates a defect that requires the vehicle to be taken out of service?

  1. The pushrod moves slightly when the brakes are applied.
  2. With the brakes released, the pushrod moves more than about one inch when pulled by hand. Correct answer.
  3. The slack adjuster is greased and free of debris.
  4. The pushrod is at a slight angle to the chamber.

Explanation

On a properly functioning automatic slack adjuster with brakes released, the pushrod should not move more than about one inch when pulled by hand. Excessive free movement means the adjuster is not maintaining the correct stroke and the brakes may be out of adjustment.