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MA CDL Air Brakes study materials
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MA CDL Air Brakes practice questions

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Question 1Air Brake System Components and Operation

In a typical MA CDL pre-trip air brake test, after the system is fully charged and the engine is shut off, what is the maximum allowable pressure loss in one minute with the brakes released and the system at governor cut-out pressure?

  1. 10 psi
  2. 1 psi
  3. 25 psi
  4. 3 psi Correct answer.

Explanation

The standard air loss test allows no more than 3 psi loss in one minute for a single (straight) vehicle with the brakes released. A leak rate above that indicates the vehicle must be tagged out of service.

Question 2Air Brake System Components and Operation

For a combination vehicle with a trailer, the one-minute air loss limit with the brakes released is:

  1. 4 psi Correct answer.
  2. 6 psi
  3. 2 psi
  4. 15 psi

Explanation

Combination vehicles are allowed up to 4 psi loss in one minute with the brakes released, because a trailer adds more connections and chambers. Straight trucks use the tighter 3 psi limit.

Question 3Air Brake System Components and Operation

During the applied-brake leakage test on a combination vehicle, the allowable pressure loss in one minute with the brakes fully applied is:

  1. 3 psi
  2. 12 psi
  3. 6 psi Correct answer.
  4. 4 psi

Explanation

With the service brakes applied and held, the combination vehicle may lose no more than 6 psi in one minute. Applied brakes pressurize more chambers, so the allowance is larger than the released test.

Question 4Air Brake System Components and Operation

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

  1. Activate after the parking brake releases
  2. Activate only when the spring brakes apply
  3. Activate at exactly 100 psi
  4. Activate before pressure drops below 55 psi Correct answer.

Explanation

The low air pressure warning must activate before air pressure falls below 55 psi. A visible warning is required; an audible warning may supplement it. The warning should occur before spring brakes apply, typically between 20 and 45 psi, giving the driver an earlier indication of dangerously low pressure.

Question 5Air Brake System Components and Operation

The governor on an air compressor serves which purpose?

  1. It meters air to the service brake chambers
  2. It drains moisture from the wet tank
  3. It limits the maximum vehicle speed
  4. It controls when the compressor loads and unloads to maintain system pressure Correct answer.

Explanation

The governor senses reservoir pressure and cuts the compressor in at cut-in pressure and out at cut-out pressure, usually around 100 and 125 psi. This keeps the system within its design range without overpressurizing.

Question 6Air Brake System Components and Operation

A driver notices the compressor will not build pressure above about 85 psi and the low pressure warning stays on. Which component is the most likely cause?

  1. A clogged trailer supply line coupler
  2. A stuck quick-release valve
  3. A leaking service brake chamber diaphragm
  4. A faulty governor or a compressor that is not loading Correct answer.

Explanation

If pressure stalls at a low value and the warning stays on, the compressor is not being commanded to load or cannot build pressure, which points to the governor or the compressor itself. Chamber and valve leaks would show up as excessive leak-down, not a hard ceiling on buildup.

Question 7Air Brake System Components and Operation

In a dual air brake system, the two circuits are best described as:

  1. Two identical circuits that always fail together
  2. Separate circuits so a failure in one still leaves braking capability in the other Correct answer.
  3. A primary circuit for the front axle only and a secondary for the horn
  4. A single circuit split only at the treadle valve

Explanation

Dual systems isolate a primary and a secondary circuit so that a loss of pressure in one does not drain the other. The vehicle retains partial braking and the warning device alerts the driver.

Question 8Air Brake System Components and Operation

After a total loss of air pressure in the primary circuit of a dual system, what should the driver expect?

  1. The steering axle will lock immediately
  2. The trailer will release its brakes automatically
  3. The spring brakes will apply and stopping distance will increase Correct answer.
  4. The service brakes will remain fully effective

Explanation

When pressure drops far enough, the spring (parking) brakes apply automatically because spring force is no longer held off by air. Whatever service braking remains will be weaker, so stopping distance increases.

Question 9Air Brake System Components and Operation

The parking/emergency brakes on an air-brake vehicle are held in the applied position by:

  1. Hydraulic pressure from the master cylinder
  2. An electric solenoid latch
  3. Mechanical spring force Correct answer.
  4. Reservoir air pressure

Explanation

Spring brakes are applied by heavy springs and released by air pressure pushing a diaphragm against the spring. Losing air pressure therefore applies the brakes, which is a fail-safe design.

Question 10Air Brake System Components and Operation

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

  1. It reduces reservoir pressure to the governor cut-in point
  2. It filters oil from the compressor discharge
  3. It holds the trailer brakes applied when the supply line is disconnected
  4. It speeds application and release of air at the brake chambers using reservoir air Correct answer.

Explanation

A relay valve uses a small control signal from the foot valve to admit reservoir air directly to nearby chambers, shortening lag time on long wheelbases and trailers. This gives faster, more balanced brake response.

Question 11Air Brake System Components and Operation

The safety valve on the air reservoir is designed to:

  1. Supply air to the trailer
  2. Release air if system pressure exceeds a set limit Correct answer.
  3. Drain water each day
  4. Signal the driver when pressure is low

Explanation

The safety valve is a pressure-relief device that opens if reservoir pressure rises above its setting, protecting the system from overpressurization. It is not a routine drain or a warning device.

Question 12Air Brake System Components and Operation

Why must the air compressor discharge air pass through an air dryer?

  1. To increase the compressor's cut-out pressure
  2. To remove moisture and contaminants before the air reaches the reservoirs Correct answer.
  3. To lubricate the brake chambers
  4. To cool the air so it expands

Explanation

Compressed air carries water vapor and oil that can freeze or corrode valves and cause brake failure. The dryer removes moisture and contaminants so clean, dry air reaches the reservoirs.

Question 13Air Brake System Components and Operation

A driver performs a compressor buildup test and finds pressure rising from 85 to 100 psi in 40 seconds. How should this be interpreted?

  1. Buildup is too fast and indicates a governor fault
  2. Buildup is too slow and the vehicle should be inspected
  3. Buildup is acceptable because it is within the typical time standard Correct answer.
  4. The test is invalid unless the engine is at idle

Explanation

The common standard is buildup from 85 to 100 psi in 45 seconds or less for a straight truck, so 40 seconds passes. Faster buildup is not a defect, and the test is normally run at governed engine speed, not idle.

Question 14Air Brake System Components and Operation

What is the purpose of the alcohol evaporator or injector sometimes fitted to air-brake systems?

  1. To raise reservoir pressure above cut-out
  2. To purge oil from the service chambers
  3. To prevent moisture in the air lines from freezing Correct answer.
  4. To lubricate the compressor valves

Explanation

Alcohol vapor carried into the system lowers the freezing point of any residual moisture, protecting valves and lines in cold weather. It does not change pressure settings or lubricate the compressor.

Question 15Air Brake System Components and Operation

The trailer hand valve (trolley valve) should be used for which purpose?

  1. Draining the trailer reservoir
  2. Releasing the trailer spring brakes
  3. Testing the trailer brakes Correct answer.
  4. Parking a combination vehicle

Explanation

The hand valve applies only the trailer service brakes and is intended for checking that trailer brakes work. It must not be used for parking, because it does not hold the vehicle and can leak down.

Question 16Air Brake System Components and Operation

When the trailer supply line is disconnected or breaks, the trailer spring brakes should:

  1. Apply only when the foot valve is pressed
  2. Release automatically
  3. Stay in their last position
  4. Apply automatically Correct answer.

Explanation

Loss of supply pressure signals the trailer spring brake valve to vent the spring chambers, applying the trailer brakes automatically. This is the tractor protection system's fail-safe behavior.

Question 17Air Brake System Components and Operation

What does the tractor protection valve do when trailer supply pressure drops too low?

  1. It releases the tractor spring brakes
  2. It bypasses the governor to speed buildup
  3. It closes the trailer supply and keeps tractor air from escaping through the broken line Correct answer.
  4. It increases pressure to the trailer service line

Explanation

The tractor protection valve isolates the tractor's air supply from the trailer when supply pressure falls, preserving tractor braking and triggering a trailer emergency application. Without it, a severed line could drain the tractor's reservoirs.

Question 18Air Brake System Components and Operation

The supply line gladhand on a tractor-trailer is normally color-coded:

  1. Green
  2. Yellow
  3. Red Correct answer.
  4. Blue

Explanation

The emergency (supply) line gladhand is red and the service line is blue. Correct color matching helps prevent cross-connection, which would cause brake malfunction.

Question 19Air Brake System Components and Operation

A driver charges the system, shuts off the engine, and finds pressure falling from 120 to 112 psi in one minute with brakes released on a combination vehicle. What is the correct action?

  1. Tag the vehicle out of service for an air leak Correct answer.
  2. Increase engine rpm to compensate
  3. Continue the trip and recheck at the next stop
  4. Pump the brakes to reseat the valves

Explanation

An 8 psi loss exceeds the 4 psi one-minute limit for a combination vehicle with brakes released, so the vehicle has a defect that must be corrected before operation. Continuing the trip risks brake failure.

Question 20Air Brake System Components and Operation

The foot valve (treadle valve) in an air brake system:

  1. Meters air pressure to the brake circuits in proportion to pedal travel Correct answer.
  2. Regulates compressor cut-in pressure
  3. Stores air for emergency braking
  4. Automatically applies the spring brakes

Explanation

The treadle valve is the driver's control that admits and modulates air to the service brake circuits according to how far the pedal is pushed. It does not store air or set compressor pressures.

Question 21Air Brake System Components and Operation

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

  1. It reverses the direction of air flow in the supply line
  2. It inverts the gladhand color coding
  3. It allows modulated service brake application even when spring brakes have applied due to low pressure Correct answer.
  4. It converts hydraulic pressure to air pressure

Explanation

The inversion valve lets service brake pressure from the foot valve reach the spring chambers during an emergency application, so the driver can still modulate braking. Without it, a low-pressure event would leave only full spring force.

Question 22Air Brake System Components and Operation

The service brake chamber converts air pressure into:

  1. Hydraulic fluid pressure
  2. Mechanical force that moves the slack adjuster and brake shoes Correct answer.
  3. Electrical current for the ABS sensors
  4. Vacuum for the booster

Explanation

Air entering the chamber pushes a diaphragm and pushrod, producing mechanical force delivered through the slack adjuster to the brake mechanism. This is the core energy conversion in an air brake system.

Question 23Air Brake System Components and Operation

Automatic slack adjusters are designed to:

  1. Lock the pushrod when pressure is released
  2. Increase chamber pressure during hard stops
  3. Maintain proper pushrod travel as brake linings wear Correct answer.
  4. Reset the governor cut-out point

Explanation

Automatic slack adjusters take up clearance as linings wear so pushrod stroke stays within limits. A driver must still check stroke during the pre-trip, because a malfunctioning adjuster can leave stroke out of adjustment.

Question 24Air Brake System Components and Operation

During a pre-trip inspection, a driver measures pushrod travel on a type 30 service chamber and finds it exceeds the adjustment limit. What is the required action?

  1. Increase governor cut-out pressure
  2. Operate normally but avoid hard stops
  3. Reduce load weight to compensate
  4. Adjust or repair the brake before operating the vehicle Correct answer.

Explanation

Excessive pushrod stroke means the brake cannot apply full force and must be adjusted or repaired before the vehicle is operated. Stroke beyond the limit is an out-of-service defect on an air-brake vehicle.

Question 25Air Brake System Components and Operation

What does the ABS warning lamp on an air-brake vehicle indicate if it stays on after the vehicle reaches about 5 mph?

  1. The trailer supply line is disconnected
  2. The ABS system has a fault and the vehicle should be inspected Correct answer.
  3. The spring brakes are fully applied
  4. The air dryer needs replacement

Explanation

A lamp that stays lit above the self-test speed signals an ABS fault, meaning the wheels may lose anti-lock protection even though base braking remains. The fault should be diagnosed and repaired.

Question 26Air Brake System Components and Operation

In a dual-circuit air brake system, the air that has already passed through the compressor and air dryer is stored where before it is routed to the individual circuits?

  1. In the treadle valve body, which doubles as a reservoir
  2. Inside the compressor crankcase, which acts as a storage tank
  3. In the supply reservoir, from which the circuits are fed and protected Correct answer.
  4. In the service brake chambers, which hold air between applications

Explanation

The compressor feeds the supply (wet) reservoir first, and the circuits draw from it through check valves and the foot valve so that a leak in one circuit cannot drain the other. The wet tank also gives moisture and oil a place to condense before air reaches the dry reservoirs.

Question 27Air Brake System Components and Operation

A driver applies the brakes and hears air escaping continuously from a chamber area even after the treadle is released. Which component has most likely failed?

  1. The safety valve on the wet tank
  2. The service brake chamber diaphragm Correct answer.
  3. The low pressure warning buzzer
  4. The air compressor governor

Explanation

A ruptured service diaphragm or a leaking chamber seal lets air escape whenever pressure is applied and often continues to leak after release. The leak is localized at the chamber, which is why the sound persists there rather than at the tank or compressor.

Question 28Air Brake System Components and Operation

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

  1. It prevents the compressor from charging the wet tank
  2. It exhausts air close to the brake chambers so they release faster Correct answer.
  3. It raises reservoir pressure above governor cut-out
  4. It limits application pressure to the trailer only

Explanation

A quick release valve is mounted near the chambers and dumps their air locally when the treadle is released, shortening the distance the exhaust air must travel back through the lines. This speeds release and reduces brake drag on long combinations.

Question 29Air Brake System Components and Operation

Which statement correctly describes the parking brake function on a vehicle equipped with spring brake chambers?

  1. Air pressure applies the brake and a spring releases it
  2. Air pressure must be applied to compress the spring and release the brake Correct answer.
  3. The brake is applied only when the service chamber is pressurized
  4. A mechanical cable applies the brake and air holds it off only on trailers

Explanation

Spring brakes are applied by heavy springs and held off by air pressure in the parking chamber. Releasing the parking brake therefore requires charging the system so that air compresses the spring, and any loss of air causes the brake to apply.

Question 30Air Brake System Components and Operation

During a static leak test the system loses 3 psi per minute with the engine off and the brakes released. What is the most appropriate interpretation?

  1. The loss is normal because air always bleeds off slowly through the compressor
  2. The loss exceeds the allowable rate and the vehicle should be placed out of service until repaired Correct answer.
  3. The loss is acceptable if the low pressure warning does not sound
  4. The loss indicates the governor cut-out pressure is set too high

Explanation

With the engine off and brakes released, a dual air system should hold pressure to within a few psi per minute, and 3 psi per minute is beyond the typical allowable leak rate. The vehicle should be tagged out of service and the leak traced before it is driven.

Question 31Air Brake System Components and Operation

What is the purpose of the check valve located between the wet tank and a dry reservoir?

  1. It signals the driver when reservoir pressure is low
  2. It regulates the pressure delivered to the service chambers
  3. It drains moisture automatically each time the compressor cycles
  4. It allows air to flow into the dry tank while preventing backflow toward the supply tank Correct answer.

Explanation

Check valves permit one-way flow so a dry reservoir stays charged even if supply pressure drops or another circuit fails. This isolation is what allows a dual system to retain braking capability after a single-circuit loss.

Question 32Air Brake System Components and Operation

A driver notices that the compressor cuts out at 120 psi and cuts back in at 100 psi, but the gauge keeps climbing past 130 psi. Which component is the most likely cause?

  1. The safety valve on the wet tank
  2. The quick release valve at the rear axle
  3. The compressor governor or its unloader mechanism Correct answer.
  4. The low pressure warning switch

Explanation

If pressure continues to rise past cut-out, the governor is not signalling the compressor to unload, so the compressor keeps pumping. The safety valve may eventually vent to protect the system, but the underlying fault is in the governor or unloader.

Question 33Air Brake System Components and Operation

What does the application of the tractor protection valve do when the trailer emergency line pressure falls to the set level?

  1. It closes the tractor lines to the trailer and vents the trailer supply line, applying the trailer spring brakes Correct answer.
  2. It disables the tractor's own service brakes to save air
  3. It opens the trailer supply line fully to keep the trailer rolling
  4. It dumps all tractor reservoir air overboard to warn following traffic

Explanation

When supply pressure drops, the tractor protection valve shuts off the tractor-to-trailer lines and vents the trailer supply line. That loss of supply pressure causes the trailer spring brakes to apply, which is the intended fail-safe behavior.

Question 34Air Brake System Components and Operation

Which condition would cause the spring brakes to apply automatically on a vehicle with a functioning system?

  1. The compressor cut-in pressure is reached
  2. The parking control valve is pulled to the park position, venting the spring chambers Correct answer.
  3. The foot valve is held down for a long period
  4. The ABS lamp turns off after the vehicle begins moving

Explanation

The parking control valve applies the spring brakes by exhausting air from the parking chambers, allowing the springs to expand. Any event that vents that air, such as a supply line break, produces the same result.

Question 35Air Brake System Components and Operation

What is the role of the double check valve in an air brake circuit that can be controlled from two sources?

  1. It divides supply pressure equally between two outlets
  2. It prevents either source from ever reaching the outlet
  3. It passes whichever of two supply pressures is higher and blocks the lower one Correct answer.
  4. It converts a service signal into a parking brake signal

Explanation

A double check valve has two inlets and one outlet and shifts to whichever inlet has the higher pressure, blocking the other. This lets a component such as a relay valve be controlled by either of two signals without allowing them to back-feed each other.

Question 36Air Brake System Components and Operation

A driver is checking the low air pressure warning and finds the buzzer sounds only after gauge pressure drops below 45 psi. How should this be evaluated?

  1. It is acceptable because the safety valve provides a backup warning
  2. It fails the requirement because the warning must activate before pressure falls below 60 psi Correct answer.
  3. It is acceptable only if the spring brakes have already applied
  4. It is acceptable because any warning above 40 psi meets the standard

Explanation

The low pressure warning device must activate before reservoir pressure drops below about 60 psi so the driver has time to react. A warning that waits until 45 psi gives too little margin and indicates a faulty switch or adjustment.

Question 37Air Brake System Components and Operation

What happens inside a relay valve when the control signal from the foot valve is applied?

  1. The relay valve converts the control signal into an electrical pulse
  2. The relay valve opens a large supply passage so reservoir air flows directly to the chambers Correct answer.
  3. The relay valve sends the control signal back to the foot valve for confirmation
  4. The relay valve vents reservoir air to atmosphere to apply the brakes

Explanation

A relay valve uses a small control signal to open a large valve fed from a nearby reservoir, so chambers fill quickly without the delay of long lines. The control signal is not returned or converted; it simply pilots the relay.

Question 38Air Brake System Components and Operation

A combination vehicle has a total of four spring brake chambers on the trailer, and the driver wants to test that the trailer parking brakes hold. Which procedure tests this correctly?

  1. Charge the system, release all parking brakes, and note whether the trailer rolls freely
  2. Apply only the foot valve and observe that the trailer wheels stop
  3. Charge the system, apply the trailer parking control, release the tractor parking brake, and try to pull against the trailer brakes Correct answer.
  4. Drain all reservoirs to zero and then check that the wheels turn by hand

Explanation

The trailer spring brakes should be applied by their own control while the tractor is free to move, so any movement shows the trailer brakes are not holding. Pulling gently against them confirms their holding ability without stressing the equipment.

Question 39Air Brake System Components and Operation

An air dryer with a faulty purge cycle is left in service. What is the most likely consequence for the brake system?

  1. Reservoir pressure rises above the governor cut-out setting
  2. The spring brakes release more slowly than normal
  3. The compressor stops building pressure entirely
  4. Moisture carries into the reservoirs and can freeze or corrode valves, reducing braking performance Correct answer.

Explanation

The purge cycle blows collected moisture and contaminants out of the dryer; if it fails, that material passes downstream into the wet and dry reservoirs. Over time this causes corrosion, frozen lines in cold weather, and valve malfunction that degrades braking.

Question 40Air Brake System Components and Operation

A vehicle with a dual air system has a complete loss of pressure in the secondary circuit. Which outcome is consistent with correct design?

  1. The primary circuit still provides service braking, and the low pressure warning alerts the driver Correct answer.
  2. The spring brakes release and cannot be applied
  3. The compressor automatically rebuilds only the failed circuit
  4. Both circuits lose pressure because they share the same reservoir

Explanation

The two circuits are separated by check valves and separate reservoirs so that a failure in one does not drain the other. The driver retains braking from the intact circuit, and the warning device signals the loss of pressure.

Question 41Dual Air Brake Systems and Safety Features

On a dual air brake system, the two separate circuits are commonly called the primary and secondary systems. Which statement correctly describes how they are arranged?

  1. Both circuits share a single reservoir but use separate foot valves.
  2. The secondary circuit is a backup that is normally empty of air until the primary fails.
  3. The primary circuit supplies the rear brakes and the secondary circuit supplies the front brakes, and each has its own reservoir. Correct answer.
  4. The primary circuit is only used when the vehicle is towing a trailer.

Explanation

A dual air brake system splits the vehicle into two independent circuits, typically a primary circuit for the rear brakes and a secondary circuit for the front brakes, each fed by its own reservoir section. Because the circuits are separate, a failure in one still leaves the other with enough stored air to stop the vehicle.

Question 42Dual Air Brake Systems and Safety Features

A dual air brake vehicle has two separate low air pressure warning indicators, one for each circuit. At what point must these warnings activate?

  1. Only after the spring brakes have fully applied.
  2. When pressure falls below 100 psi in both circuits at once.
  3. Before pressure in either circuit drops below 60 psi. Correct answer.
  4. Whenever the compressor cycles off.

Explanation

Federal requirements call for a warning to activate before the air pressure in either circuit drops below 60 psi. Two separate warning devices are required so the driver is alerted to a loss in either half of the system.

Question 43Dual Air Brake Systems and Safety Features

During a pre-trip inspection, you perform the static leak test on a dual air brake system. Which result indicates the system is within acceptable limits?

  1. No pressure loss at all is required to pass.
  2. Pressure loss of less than 20 psi per minute.
  3. Pressure loss of less than 3 psi per minute with the engine off and brakes released. Correct answer.
  4. Any steady drop in pressure as long as it stops before reaching 60 psi.

Explanation

A single vehicle should lose less than 3 psi in one minute with the engine off and the brakes released; a combination vehicle allows less than 4 psi per minute. A steady loss beyond those limits indicates a leak that must be repaired.

Question 44Dual Air Brake Systems and Safety Features

You are driving a dual air brake vehicle and the primary circuit suddenly loses pressure. What should you expect from the secondary circuit?

  1. It locks the front wheels until the vehicle stops.
  2. It has no effect because both circuits share the same relay valve.
  3. It immediately releases all its air to balance the system.
  4. It continues to supply its own brakes, allowing a controlled stop with longer stopping distance. Correct answer.

Explanation

The purpose of the dual system is that the intact circuit keeps working after the other fails. The driver retains braking on the unaffected circuit but should expect longer stopping distances and must pull off the road safely.

Question 45Dual Air Brake Systems and Safety Features

What is the function of the safety valve installed in an air brake system?

  1. It releases air to the atmosphere if system pressure rises above a preset limit. Correct answer.
  2. It automatically drains moisture from the reservoirs.
  3. It prevents the compressor from running when the engine is cold.
  4. It limits the maximum braking force applied by the foot valve.

Explanation

The safety valve is a pressure relief device that opens to vent excess air if pressure climbs above its set point, protecting components from over-pressurization. It is typically set higher than the governor cut-out pressure.

Question 46Dual Air Brake Systems and Safety Features

The governor on an air brake system serves which purpose?

  1. It limits the speed of the vehicle when air pressure is low.
  2. It regulates the pressure delivered to the trailer gladhands only.
  3. It controls when the compressor builds air and when it stops, based on reservoir pressure. Correct answer.
  4. It meters air to the front brake chambers during a stop.

Explanation

The governor senses reservoir pressure and signals the compressor to load when pressure falls to the cut-in point and to unload at the cut-out point. This keeps system pressure within its designed operating range.

Question 47Dual Air Brake Systems and Safety Features

You are checking governor cut-in and cut-out pressures on a dual air brake vehicle. Which pair of readings is typical for a properly functioning system?

  1. Cut-in near 150 psi and cut-out near 175 psi.
  2. Cut-in near 60 psi and cut-out near 80 psi.
  3. Cut-in near 100 psi and cut-out near 125 psi. Correct answer.
  4. Cut-in near 125 psi and cut-out near 100 psi.

Explanation

A typical governor allows the compressor to resume building air around 100 psi and to stop around 125 psi. These values keep the system charged well above the 60 psi warning threshold.

Question 48Dual Air Brake Systems and Safety Features

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

  1. To increase system pressure above the governor cut-out.
  2. To lubricate the brake chambers internally.
  3. To remove moisture and prevent it from freezing or corroding components. Correct answer.
  4. To cool the compressed air before it reaches the foot valve.

Explanation

Moisture in compressed air can freeze in valves and lines or cause corrosion, so an evaporator or dryer removes water vapor. Keeping the air dry protects the valves and helps prevent winter brake failures.

Question 49Dual Air Brake Systems and Safety Features

A vehicle has spring brakes. What happens to them when system air pressure drops to a low level, such as around 20 to 45 psi?

  1. The brakes remain in their last position until the driver presses the pedal.
  2. The brakes release because the chambers lose holding force.
  3. The springs apply the brakes automatically as air pressure is lost. Correct answer.
  4. The brakes apply only if the parking brake valve is pulled.

Explanation

Spring brakes use heavy springs to apply the brakes and air pressure to hold them released. When pressure drops below the spring's release point, the springs push the brakes on automatically, providing a fail-safe stop.

Question 50Dual Air Brake Systems and Safety Features

Before driving, you must ensure the spring brakes will hold. Which test confirms the parking brake function on a dual air brake vehicle?

  1. Release the parking brake and check that the vehicle rolls freely.
  2. Build pressure to cut-out and listen for the safety valve to vent.
  3. With the system charged, apply the parking brake and try to move forward gently to confirm the vehicle holds. Correct answer.
  4. Drain all air and confirm the wheels turn by hand.

Explanation

A parking brake test confirms the spring brakes engage and hold the vehicle with the system charged. Gently attempting to move with the brake applied verifies that the brakes are actually holding.