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Saskatchewan Air Brake study materials
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Saskatchewan Air Brake practice questions

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First 50 of 199 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

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Question 1Air Brake System Fundamentals

In a typical Westinghouse-style air brake system used on Saskatchewan heavy vehicles, what is the primary source of compressed air for the brake system?

  1. An engine-driven compressor Correct answer.
  2. A hydraulic accumulator
  3. A vacuum booster
  4. A manual hand pump

Explanation

The air brake system is supplied by an engine-driven compressor that charges the reservoirs through a governor-controlled cycle. Hydraulic, vacuum, and manual sources do not produce the sustained high-pressure air volume required for air brakes.

Question 2Air Brake System Fundamentals

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

  1. It regulates the rate of air pressure applied to the brake chambers
  2. It controls compressor loading and unloading to maintain reservoir pressure within a set range Correct answer.
  3. It meters the flow of air to the trailer supply line
  4. It automatically applies the spring brakes when pressure drops

Explanation

The governor senses reservoir pressure and signals the compressor to load (pump) or unload (idle) so pressure stays within a designed cut-in and cut-out range. It does not proportion brake application force, apply spring brakes, or meter trailer supply flow.

Question 3Air Brake System Fundamentals

A driver notices the low air pressure warning activates at about 60 psi. What does this warning indicate?

  1. The spring brakes have fully released and the system is normal
  2. Reservoir pressure has fallen into a range where the brakes may not be fully effective and the vehicle should be stopped safely Correct answer.
  3. The trailer has been disconnected from the supply line
  4. The compressor has reached its cut-out pressure and stopped pumping

Explanation

The low pressure warning must activate before pressure drops below the level needed for safe braking, commonly around 60 psi. It alerts the driver to stop safely and investigate rather than continue driving.

Question 4Air Brake System Fundamentals

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

  1. To lubricate the brake chambers
  2. To reduce engine load during braking
  3. To increase reservoir pressure above cut-out
  4. To remove moisture and prevent ice and corrosion in the air system Correct answer.

Explanation

Moisture in compressed air can freeze in cold weather and corrode valves, so an evaporator or air dryer removes or controls water vapor. Saskatchewan winter operation makes moisture control especially important for reliable brake performance.

Question 5Air Brake System Fundamentals

In a dual circuit air brake system, why are the reservoirs divided into separate circuits?

  1. To permit the trailer to use the tractor's primary circuit
  2. To allow the compressor to fill only one circuit at a time
  3. To reduce the total air volume needed for braking
  4. So a failure in one circuit still leaves a protected supply for part of the braking system Correct answer.

Explanation

Dual circuits isolate the front and rear (or primary and secondary) systems so a single failure does not leave the vehicle without braking. Check valves and the foot valve design protect the remaining charged circuit.

Question 6Air Brake System Fundamentals

What component converts air pressure into the mechanical force that applies the foundation brakes?

  1. The quick release valve
  2. The brake chamber Correct answer.
  3. The relay valve
  4. The supply reservoir

Explanation

The brake chamber contains a diaphragm and pushrod that convert air pressure into mechanical force to apply the brake. Relay and quick release valves control timing and release, while reservoirs store air.

Question 7Air Brake System Fundamentals

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

  1. Excessive pressure caused by a governor or compressor malfunction Correct answer.
  2. Freezing of moisture in the reservoir
  3. Loss of pressure caused by a leaking line
  4. Backflow from the trailer supply line

Explanation

The safety valve is a pressure relief device that opens if reservoir pressure exceeds a safe limit, such as when the governor fails and the compressor keeps pumping. It protects the system from over-pressurization damage.

Question 8Air Brake System Fundamentals

What is the normal function of the drain cocks on air reservoirs?

  1. To test the low pressure warning switch
  2. To remove accumulated water and oil from the bottom of the reservoir Correct answer.
  3. To release air pressure to the brake chambers
  4. To balance pressure between the two circuits

Explanation

Water and oil collect at the lowest point of a reservoir, so drain cocks are opened to blow off contaminants. Regular draining helps prevent corrosion and freeze-ups in the air system.

Question 9Air Brake System Fundamentals

A driver applies the foot valve and the brakes apply, but release is noticeably slow on a tractor axle. Which component most directly affects how quickly air exhausts from the brake chambers?

  1. The compressor
  2. The quick release valve Correct answer.
  3. The governor
  4. The supply reservoir

Explanation

Quick release valves sense falling control pressure and exhaust chamber air close to the axle, shortening release time. Slow release often points to a restricted or faulty quick release valve or exhaust port.

Question 10Air Brake System Fundamentals

On a tractor-trailer combination, what is the role of the tractor protection valve?

  1. It closes the tractor supply and control lines if trailer pressure is lost or the trailer separates Correct answer.
  2. It automatically applies the tractor spring brakes during a normal stop
  3. It limits the maximum pressure delivered to the trailer brake chambers
  4. It balances air pressure between the tractor primary and secondary circuits

Explanation

The tractor protection valve protects the tractor air supply by closing off the trailer lines when pressure falls or the trailer breaks away. This preserves tractor braking and prevents a rapid loss of tractor air.

Question 11Air Brake System Fundamentals

What happens when the trailer supply line is disconnected or breaks away from the tractor?

  1. The trailer emergency spring brakes apply automatically as trailer supply pressure is lost Correct answer.
  2. The trailer service brakes release and the trailer rolls freely
  3. The trailer reservoirs remain fully charged and unaffected
  4. The tractor compressor immediately stops pumping

Explanation

Loss of supply line pressure signals the trailer emergency relay valve to apply the trailer spring brakes, providing a fail-safe stop. This is the breakaway protection built into the trailer air system.

Question 12Air Brake System Fundamentals

A driver performs a static leak test and finds pressure drops faster than the allowable limit in the applied brake position. What is the most appropriate interpretation?

  1. The low pressure warning is malfunctioning
  2. The governor cut-out is set too low
  3. There is an air leak in the service brake circuit that must be repaired before operation Correct answer.
  4. The compressor is overcharging the system

Explanation

Excessive pressure loss with brakes applied indicates a leak in the service side of the system, such as a chamber, hose, or valve. The vehicle should be removed from service until the leak is corrected.

Question 13Air Brake System Fundamentals

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

  1. To invert the trailer supply and control line functions
  2. To convert hydraulic pressure to air pressure
  3. To reverse the direction of air flow in the supply line
  4. To allow a modulated spring brake application when service brake pressure is lost Correct answer.

Explanation

The inversion valve provides a controlled, modulated application of spring brakes when service air is lost, rather than an abrupt full application. This improves stopping control during an emergency.

Question 14Air Brake System Fundamentals

What is the function of the relay valve in a tractor rear brake circuit?

  1. It applies the spring brakes when the parking valve is set
  2. It uses reservoir air to apply the brakes quickly in response to a control signal Correct answer.
  3. It drains moisture from the rear reservoir
  4. It reduces reservoir pressure to the governor cut-in point

Explanation

A relay valve shortens application and release time by supplying air directly from a nearby reservoir when signaled by the foot valve. This improves brake timing on axles far from the foot valve.

Question 15Air Brake System Fundamentals

During a pre-trip inspection, the driver builds system pressure and then shuts off the engine. Which observation confirms the system is holding pressure adequately in the charged, unapplied condition?

  1. The compressor cycles on with the engine off
  2. Pressure drop stays within the allowable limit for the specified time Correct answer.
  3. Pressure rises above cut-out and holds
  4. The low pressure warning sounds immediately

Explanation

With the system charged and brakes released, pressure should remain stable within a small allowable loss over the test period. A larger drop indicates a leak that must be fixed before the trip.

Question 16Air Brake System Fundamentals

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

  1. To regulate the compressor cut-in pressure
  2. To provide additional service braking force at high pressure
  3. To apply the brakes mechanically when air pressure is released Correct answer.
  4. To store air for the trailer supply line

Explanation

A large spring applies the brakes when air pressure is removed, providing parking and emergency braking. Air pressure compresses the spring to release the brakes during normal operation.

Question 17Air Brake System Fundamentals

A vehicle with a dual circuit system loses pressure in the primary circuit. What is the expected result when the driver applies the foot valve?

  1. The spring brakes apply fully and cannot be released
  2. All braking is lost immediately
  3. The trailer brakes apply but the tractor brakes do not
  4. The secondary circuit still provides braking to its protected wheels Correct answer.

Explanation

Dual circuit design isolates the circuits so the secondary circuit can still deliver braking if the primary fails. The driver should still have partial braking to stop safely.

Question 18Air Brake System Fundamentals

Why must the compressor be capable of building system pressure from zero within a reasonable time during a check?

  1. To test the trailer breakaway system
  2. To measure the spring brake release pressure
  3. To verify the safety valve opens at cut-out
  4. To confirm the compressor and governor can recover pressure needed for braking after a loss Correct answer.

Explanation

A build-up time test confirms the compressor can restore adequate pressure after a loss, which is essential for safe braking. Slow build-up may indicate compressor, governor, or leak problems.

Question 19Air Brake System Fundamentals

What is the function of the check valve installed between the compressor and the supply reservoir?

  1. It signals the governor to unload the compressor
  2. It prevents air from flowing back from the reservoir to the compressor Correct answer.
  3. It removes oil from the compressed air
  4. It limits reservoir pressure to the cut-out setting

Explanation

The check valve allows air to flow from the compressor into the reservoir but blocks backflow when the compressor unloads. This keeps stored air from escaping back through the compressor.

Question 20Air Brake System Fundamentals

A driver finds that the spring brakes will not release after the reservoir is charged. Which condition is the most likely cause?

  1. Excessive compressor cut-out pressure
  2. A plugged reservoir drain cock
  3. Insufficient pressure reaching the spring brake release circuit Correct answer.
  4. A faulty low pressure warning switch

Explanation

Spring brakes release only when enough air pressure reaches the spring brake chambers through the parking valve and release circuit. A restriction or valve fault in that circuit prevents release.

Question 21Air Brake System Fundamentals

In cold Saskatchewan conditions, a driver reports that brakes drag or release slowly after a cold soak. Which air system condition is the most likely explanation?

  1. Moisture in the system has frozen in a valve or line, restricting air flow Correct answer.
  2. The safety valve is opening too early
  3. The governor cut-out pressure is set too high
  4. The compressor is producing too much air volume

Explanation

Freezing moisture can block or restrict valves and lines, causing slow release or dragging brakes in cold weather. Proper draining, drying, and alcohol evaporation help prevent this condition.

Question 22Air Brake System Fundamentals

A technician observes that reservoir pressure rises to cut-out, then immediately falls well below cut-in and the compressor reloads repeatedly. Which fault best explains this cycling?

  1. A low pressure warning switch set too high
  2. A correctly operating governor with normal hysteresis
  3. A safety valve set above cut-out pressure
  4. A leaking check valve or governor signal line allowing pressure to bleed back Correct answer.

Explanation

Rapid reloading after cut-out suggests pressure is escaping back through a leaking check valve or a faulty governor signal line. The governor then senses low pressure and reloads the compressor prematurely.

Question 23Air Brake System Fundamentals

During a tug test and breakaway check, the trailer spring brakes apply when the supply line is pulled. What does this confirm?

  1. The tractor protection valve is stuck open
  2. The trailer reservoir is overcharged
  3. The trailer service brake chambers are leaking
  4. The trailer emergency relay valve and spring brake system are providing fail-safe protection Correct answer.

Explanation

A successful breakaway test confirms that loss of supply pressure triggers the trailer emergency relay valve to apply the spring brakes. This verifies the fail-safe protection works as designed.

Question 24Air Brake System Fundamentals

A dual circuit system is designed so that a failure of one circuit does not drain the other. Which component arrangement accomplishes this protection?

  1. A relay valve that combines both circuits into one delivery
  2. A governor that feeds both circuits through one line
  3. Check valves and a foot valve with separate inlet and delivery sections for each circuit Correct answer.
  4. A single reservoir shared by both circuits without check valves

Explanation

Check valves keep each circuit charged independently, and the dual foot valve delivers from the intact circuit if the other loses pressure. This isolation preserves partial braking after a failure.

Question 25Air Brake System Fundamentals

A driver reports that the low pressure warning does not activate when pressure is drained below the specified threshold. What is the most serious consequence of operating with this fault?

  1. The trailer supply line will over-pressurize
  2. The compressor will overcharge the reservoirs
  3. The spring brakes will release too quickly
  4. The driver may continue driving without warning while braking capability is dangerously reduced Correct answer.

Explanation

The low pressure warning is a critical alert that braking capability is falling; if it fails, the driver may not realize the danger and continue driving. The fault must be repaired before operation.

Question 26Air Brake System Fundamentals

In a typical Saskatchewan heavy truck air brake system, what is the primary source of compressed air?

  1. The exhaust brake
  2. The spring brake chambers
  3. The engine-driven compressor Correct answer.
  4. The trailer supply line

Explanation

The compressor, usually gear-driven off the engine, is the only component that generates compressed air for the system. Other components store, control, or use that air but do not produce it. The spring brake chambers consume air to release, and the trailer supply line only delivers air already produced by the tractor.

Question 27Air Brake System Fundamentals

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

  1. To remove moisture and contaminants from compressed air before it reaches the reservoirs Correct answer.
  2. To cool the compressed air below freezing before storage
  3. To lubricate the brake chambers with atomized oil
  4. To increase the compressor's delivery pressure above governor cut-out

Explanation

The air dryer contains a desiccant that absorbs water vapor and traps oil and dirt so that reservoirs receive drier, cleaner air. This reduces corrosion and prevents ice formation in lines during Saskatchewan winters. It does not raise pressure, lubricate chambers, or intentionally chill the air.

Question 28Air Brake System Fundamentals

What does the governor do on an air brake system?

  1. It limits vehicle speed when the spring brakes are applied
  2. It unloads the compressor when reservoir pressure reaches cut-out and reloads it at cut-in Correct answer.
  3. It controls the rate of air loss during a breakaway event
  4. It regulates the pressure delivered to the trailer service line only

Explanation

The governor senses reservoir pressure and signals the compressor's unloader mechanism to stop pumping at cut-out pressure and resume at cut-in pressure. This maintains system pressure within the designed operating range. It does not govern road speed or meter trailer service air; those functions belong to other valves.

Question 29Air Brake System Fundamentals

What is the function of the supply reservoir in a dual circuit air brake system?

  1. It stores air that has been conditioned and provides a source for both the primary and secondary circuits Correct answer.
  2. It acts as a surge tank that increases compressor cut-out pressure
  3. It holds air only for the trailer emergency line
  4. It supplies air directly to the spring brake chambers without going through a valve

Explanation

The supply reservoir receives air from the compressor and dryer and feeds the primary and secondary circuits through check valves. It acts as the central storage point for conditioned air. It does not feed spring brakes directly, nor does it change the governor's pressure settings.

Question 30Air Brake System Fundamentals

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

  1. To automatically apply the spring brakes when pressure is low
  2. To prevent the compressor from building pressure above cut-out
  3. To alert the driver when system pressure falls below a safe operating level Correct answer.
  4. To indicate when the air dryer desiccant needs replacement

Explanation

The low pressure warning device, typically a buzzer and light, activates when reservoir pressure drops below the specified threshold so the driver can stop safely. It does not itself apply the brakes or control compressor output. Desiccant condition is not monitored by this device.

Question 31Air Brake System Fundamentals

What is the function of the quick release valve in a truck air brake circuit?

  1. It isolates a failed circuit from the rest of the system
  2. It limits the maximum pressure in the service line
  3. It speeds up the application of the brakes by boosting pressure
  4. It exhausts air from the brake chambers rapidly when the brake valve is released Correct answer.

Explanation

The quick release valve is located near the brake chambers and opens an exhaust port when service line pressure drops, allowing chamber air to escape quickly for a faster release. It does not boost application pressure, isolate circuits, or limit service pressure.

Question 32Air Brake System Fundamentals

What is the role of the tractor protection valve when the trailer supply line is intact and charged?

  1. It vents trailer supply air to atmosphere continuously
  2. It permanently blocks air to the trailer until the foot valve is pressed
  3. It reduces trailer supply pressure to a fixed low value at all times
  4. It allows air to pass to the trailer and remains open until supply pressure drops to a low level Correct answer.

Explanation

With normal supply pressure, the tractor protection valve stays open so the trailer can be charged and controlled. If supply pressure falls below a safe level, the valve closes to protect the tractor system and signal the trailer to apply its spring brakes. It does not permanently block or continuously vent trailer air.

Question 33Air Brake System Fundamentals

What is the purpose of the service line in a tractor-trailer air brake system?

  1. It provides the emergency air that holds off the trailer spring brakes
  2. It carries the variable control pressure that signals the trailer to apply or release its service brakes Correct answer.
  3. It supplies constant air for charging the trailer reservoirs
  4. It exhausts air from the trailer brake chambers during release

Explanation

The service line is the control line from the tractor foot valve to the trailer; its pressure varies with driver demand to apply or release trailer service brakes. The supply line, by contrast, charges trailer reservoirs and holds off spring brakes. Exhausting chamber air is done by trailer relay and quick release valves.

Question 34Air Brake System Fundamentals

A driver notices that the air compressor runs continuously and system pressure never reaches governor cut-out. Which condition is the most likely cause?

  1. An air dryer with a saturated desiccant bed
  2. A significant air leak or a faulty unloader Correct answer.
  3. A governor set to cut out at a lower pressure
  4. A quick release valve stuck in the closed position

Explanation

If the compressor cannot reach cut-out, air is either escaping as fast as it is produced or the unloader is not allowing the compressor to pump. A large leak or a failed unloader fits this symptom. A saturated dryer restricts flow but does not by itself keep the compressor running continuously if the rest of the system is tight.

Question 35Air Brake System Fundamentals

During a pre-trip inspection, the driver drains the supply reservoir and hears a continuous flow of air from the drain cock after the tank should be empty. What does this indicate?

  1. The low pressure warning buzzer is defective
  2. The compressor is still pumping air into the reservoir or the drain cock is leaking Correct answer.
  3. The safety valve is stuck open
  4. The trailer supply line is disconnected

Explanation

A continuous flow after the reservoir should be empty suggests the compressor is still supplying air or the drain cock itself is not sealing. A stuck safety valve would vent at its set pressure, not continuously at low pressure. A disconnected trailer line would not cause air to flow from the tractor supply reservoir drain.

Question 36Air Brake System Fundamentals

A driver builds system pressure to cut-out, shuts off the engine, and observes the gauge. After one hour, pressure has dropped from 120 psi to 80 psi with no brake applied. What is the most appropriate interpretation?

  1. There is an excessive leak in the system that requires repair before operation Correct answer.
  2. This is normal because air pressure always bleeds off over time
  3. The governor cut-in setting is too high
  4. The low pressure warning device is incorrectly adjusted

Explanation

With the engine off and brakes released, system pressure should remain stable or drop only very slowly. A 40 psi loss in one hour indicates a leak that must be located and repaired. Normal systems do not lose pressure at that rate, and governor or warning settings do not cause static pressure loss.

Question 37Air Brake System Fundamentals

A technician finds that the spring brakes on a tractor will not release even though system pressure is at normal cut-out. Which component is the most likely cause?

  1. A quick release valve installed backwards on the service line
  2. A leaking service line to the trailer
  3. A governor stuck in the unloaded position
  4. A faulty spring brake control valve not supplying air to the chambers Correct answer.

Explanation

Spring brakes release when air is delivered to their spring brake chambers through the spring brake control valve. If that valve fails to supply air, the brakes stay applied despite normal reservoir pressure. A leaking trailer service line or a backwards quick release valve would affect service brake performance, not hold off the tractor spring brakes.

Question 38Air Brake System Fundamentals

What is the purpose of the relay valve in a trailer air brake system?

  1. It automatically applies the trailer spring brakes when the service line pressure is lost
  2. It charges the trailer reservoir from the tractor supply line
  3. It reduces the pressure in the trailer supply line to protect the trailer reservoir
  4. It uses a small control signal from the service line to deliver a large volume of air from the trailer reservoir to the brake chambers Correct answer.

Explanation

The trailer relay valve is a volume booster: a modest control pressure from the service line opens it to admit reservoir air directly to the trailer brake chambers. This shortens application time on long trailer lines. It does not charge the reservoir or control the supply line; those are functions of other valves.

Question 39Air Brake System Fundamentals

A driver performs a tug test on a tractor-trailer and finds that the trailer wheels do not hold when the tractor pulls forward with the trailer brakes set. What is the most likely explanation?

  1. The tractor protection valve is stuck open
  2. The trailer spring brakes are not applying or are out of adjustment Correct answer.
  3. The trailer supply line is overcharged
  4. The quick release valve is leaking

Explanation

A successful tug test requires the trailer spring brakes to hold the trailer stationary. If the trailer moves, the spring brakes are not applying effectively, possibly due to a fault in the emergency circuit or excessive slack adjuster travel. A stuck-open protection valve or overcharged supply line would not by itself prevent spring brake application.

Question 40Air Brake System Fundamentals

In a dual circuit air brake system, what is the purpose of the check valves between the supply reservoir and the primary and secondary reservoirs?

  1. They allow each circuit to be charged from the supply reservoir while preventing loss of the other circuit's air through a failure Correct answer.
  2. They provide a path for air to escape when the compressor unloads
  3. They limit the maximum pressure in each circuit to a preset value
  4. They automatically drain moisture from each reservoir

Explanation

Check valves permit air to flow from the supply reservoir into each circuit but block reverse flow, so a failure in one circuit does not drain the other. They do not regulate pressure, drain moisture, or vent air during unloading. This isolation is a key feature of dual circuit safety.

Question 41Air Brake System Fundamentals

A driver reports that the low pressure warning buzzer does not sound when system pressure is drained below the specified threshold. What is the first check a technician should perform?

  1. Replace the air dryer desiccant
  2. Adjust the governor cut-in pressure
  3. Verify the warning device's electrical connections and pressure switch operation Correct answer.
  4. Replace the compressor

Explanation

The warning device depends on a pressure switch and electrical circuit; a failed switch, blown fuse, or bad ground will prevent it from operating. The compressor, governor, and dryer are unrelated to whether the warning sounds at low pressure. Checking the circuit and switch is the logical first step.

Question 42Air Brake System Fundamentals

What does the alcohol evaporator, where fitted, do in an air brake system?

  1. It lubricates the brake chamber diaphragms
  2. It increases the compressor's output pressure
  3. It cleans the air of oil and carbon particles
  4. It introduces alcohol vapor into the air system to help prevent ice formation in valves and lines Correct answer.

Explanation

An alcohol evaporator adds alcohol vapor to the compressed air so that any moisture present is less likely to freeze in lines and valves during cold weather. It is not a filter, does not raise pressure, and is not intended to lubricate chambers. In Saskatchewan, this function is especially relevant in winter.

Question 43Air Brake System Fundamentals

A tractor has a dual circuit system. If the primary circuit loses pressure, what should the driver expect when applying the foot valve?

  1. The trailer service brakes will apply at full pressure regardless of foot valve input
  2. The spring brakes will automatically release
  3. The secondary circuit will still provide braking to its associated wheels, though stopping distance may increase Correct answer.
  4. All braking will be lost immediately

Explanation

Dual circuit design isolates the two service circuits so that a failure in one leaves the other operational. The driver retains braking on the wheels served by the intact circuit, with reduced overall braking effort. Total loss of braking or automatic spring brake release does not occur from a single circuit failure.

Question 44Air Brake System Fundamentals

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

  1. It allows air to enter the reservoir but prevents backflow
  2. It vents excess pressure to atmosphere if the governor fails and pressure exceeds a safe limit Correct answer.
  3. It maintains a constant pressure in the reservoir regardless of compressor output
  4. It automatically drains water from the reservoir

Explanation

The safety valve is a pressure relief device that opens if reservoir pressure rises above its setting, protecting the tank and system from overpressure. It does not regulate normal pressure, act as a check valve, or drain moisture. Its role is purely protective.

Question 45Air Brake System Fundamentals

A driver in Saskatchewan notices that after a cold night, the brakes on the truck release very slowly and the wheels drag. Which air system condition is the most likely cause?

  1. The safety valve is leaking
  2. Moisture in the system has frozen in a valve or line, restricting air flow Correct answer.
  3. The compressor is worn and cannot build pressure
  4. The governor cut-out is set too high

Explanation

Slow release and dragging after a cold soak strongly suggest ice blocking an exhaust or supply passage in a valve or line. Moisture enters with compressed air and condenses; if not removed by draining and drying, it freezes in cold weather. A worn compressor or high cut-out would affect pressure build, not release speed.

Question 46Air Brake System Fundamentals

What is the purpose of the front brake limiting valve (where fitted) on a heavy truck?

  1. It increases front braking force to shorten stopping distance
  2. It isolates the front circuit during a rear circuit failure
  3. It reduces the air pressure delivered to the front brake chambers under certain conditions to help prevent wheel lock-up Correct answer.
  4. It automatically applies the front brakes when the trailer breaks away

Explanation

The front brake limiting valve, sometimes called a 'slippery surface' valve, reduces front brake application pressure to lessen the chance of front wheel lock-up on low-friction surfaces. It does not increase braking force, apply brakes on breakaway, or isolate circuits. Its effect is to limit, not boost, front brake pressure.

Question 47Air Brake System Fundamentals

A technician is testing the air system and finds that pressure rises to cut-out, then immediately falls well below cut-in before the compressor restarts. Which condition best explains this?

  1. A quick release valve stuck open
  2. A faulty check valve allowing air to bleed back through the compressor Correct answer.
  3. A governor set to cut in at a higher pressure than cut-out
  4. An air dryer with a restricted purge orifice

Explanation

If the check valve between the compressor and the supply reservoir leaks, air bleeds back through the compressor when it unloads, causing a rapid pressure drop after cut-out. The governor then restarts the compressor at cut-in. A stuck quick release valve would affect brake release, and a restricted purge orifice would not cause rapid backflow.

Question 48Air Brake System Fundamentals

In a dual circuit air brake system, which component arrangement ensures that a failure in one service circuit does not drain the other circuit's reservoir?

  1. Check valves between the supply reservoir and each circuit reservoir Correct answer.
  2. A single reservoir serving both circuits
  3. A safety valve on each brake chamber
  4. A quick release valve on each axle

Explanation

Check valves allow each circuit reservoir to be charged from the supply reservoir while preventing reverse flow, so a failure in one circuit cannot empty the other. A single shared reservoir would not provide isolation. Quick release and safety valves serve different purposes and do not isolate circuits.

Question 49Air Brake System Fundamentals

A driver reports that the trailer spring brakes apply immediately when the trailer supply line is connected to the tractor, even before the foot valve is pressed. What is the most likely cause?

  1. The tractor quick release valve is stuck open
  2. The trailer emergency relay valve is not receiving supply pressure or is faulty Correct answer.
  3. The governor cut-out is set too low
  4. The trailer service line is leaking

Explanation

Trailer spring brakes are held off by supply pressure delivered through the emergency relay valve. If that valve does not receive or pass supply pressure, the spring brakes remain applied even when the line is connected. A leaking service line or a stuck quick release valve would affect service brakes, not spring brake hold-off.

Question 50Air Brake System Fundamentals

During a static leak test with the brakes applied, a technician finds that pressure drops faster than the allowable limit, but only when the foot valve is held in the applied position. Which component is the most likely source of the leak?

  1. The supply reservoir drain cock
  2. The compressor discharge line
  3. The governor signal line
  4. A service brake chamber diaphragm or a service line fitting Correct answer.

Explanation

A leak that appears only when the brakes are applied points to the service brake circuit downstream of the foot valve, such as a chamber diaphragm or a service line fitting. Compressor, reservoir drain, and governor components are in the supply side and would leak regardless of brake application. Isolating the leak to the applied circuit narrows the cause.