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NM CDL Air Brakes study materials
NM CDL Air Brakes study resources

NM CDL Air Brakes practice questions

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Question 1Air Brake System Components and Operation

On a CDL vehicle, the air compressor is normally driven by which component?

  1. The rear axle through a driveshaft coupling
  2. An electric motor that runs whenever the ignition is on
  3. The engine through a belt or gear drive Correct answer.
  4. The starter motor through a reduction gear

Explanation

The compressor is engine-driven by a belt or gear so it builds air whenever the engine runs. It has no separate electric or axle drive, and the starter only cranks the engine.

Question 2Air Brake System Components and Operation

What is the purpose of the air compressor governor?

  1. To control the compressor so it maintains air pressure within a set range Correct answer.
  2. To drain moisture from the supply tank automatically
  3. To limit the maximum pressure delivered to the brake chambers
  4. To signal the driver when pressure falls below the warning level

Explanation

The governor senses supply-tank pressure and engages or disengages the compressor so pressure stays between the cut-in and cut-out settings. It does not drain tanks, limit chamber pressure, or sound the low-pressure warning.

Question 3Air Brake System Components and Operation

During normal operation, at what point does the governor cut out the compressor?

  1. When supply pressure reaches the cut-out setting, roughly 120 to 140 psi Correct answer.
  2. When the low-pressure warning buzzer sounds
  3. When the service brakes are first applied
  4. When the parking brake is released

Explanation

The governor stops the compressor from pumping once supply pressure reaches the cut-out setting, commonly around 120 to 140 psi. It restarts pumping at the lower cut-in setting as pressure falls.

Question 4Air Brake System Components and Operation

What does the supply reservoir (wet tank) do in an air brake system?

  1. It supplies air only to the front brake chambers
  2. It collects air directly from the compressor and holds it before it reaches the other tanks Correct answer.
  3. It stores air that has already passed through the foot valve
  4. It holds air only for the trailer emergency brake

Explanation

The supply or wet tank receives air straight from the compressor and is the first place moisture and oil drop out. From there air moves to the dry tanks that feed the brake circuits.

Question 5Air Brake System Components and Operation

What is the function of a safety valve on an air tank?

  1. It releases air if tank pressure rises above a preset limit Correct answer.
  2. It regulates the pressure sent to the trailer
  3. It automatically drains water each time the brakes are applied
  4. It closes off the tank if the compressor fails

Explanation

The safety valve is a pressure-relief device that opens if tank pressure exceeds its setting, protecting the tank from overpressure. It does not isolate the tank, drain water, or regulate trailer pressure.

Question 6Air Brake System Components and Operation

What is the purpose of the alcohol evaporator sometimes fitted to an air brake system?

  1. To increase compressor output at high altitude
  2. To lubricate the brake chambers
  3. To help prevent moisture in the system from freezing Correct answer.
  4. To raise the cut-out pressure of the governor

Explanation

An alcohol evaporator introduces alcohol vapor into the air system to lower the freezing point of any moisture present. It does not boost compressor output, lubricate chambers, or change governor settings.

Question 7Air Brake System Components and Operation

In a dual air brake system, how are the two circuits normally arranged?

  1. One circuit for the parking brake and one for the warning buzzer
  2. One circuit for the front axle and one for the rear axle, each with its own reservoir Correct answer.
  3. One circuit for service brakes and one for the compressor
  4. One circuit for the tractor and one for the engine brake

Explanation

A dual system splits the vehicle into two separate circuits, typically front and rear, each with its own reservoir and check valve. If one circuit fails, the other can still stop the vehicle.

Question 8Air Brake System Components and Operation

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

  1. It meters air pressure to the brake circuits in proportion to how far the pedal is pushed Correct answer.
  2. It holds the trailer brakes applied when the tractor is parked
  3. It sets the governor cut-in pressure
  4. It drains the wet tank when the pedal is released

Explanation

The foot valve is the driver's service brake control; pushing the treadle meters air pressure to the relay valves or chambers. The harder and farther it is pushed, the greater the pressure delivered.

Question 9Air Brake System Components and Operation

A driver notices that the low-pressure warning buzzer and light come on while driving. What does this indicate?

  1. The system pressure has dropped to an unsafe level Correct answer.
  2. The compressor has reached its cut-out pressure
  3. The parking brake is fully released
  4. The trailer has been disconnected from the tractor

Explanation

The warning buzzer and light are designed to activate when system air pressure falls to an unsafe level, generally around 60 psi or below. The driver should stop and investigate rather than continue at speed.

Question 10Air Brake System Components and Operation

During a pre-trip check, pressure builds very slowly and the compressor never reaches cut-out. Which condition is the most likely cause?

  1. A governor set too high
  2. A safety valve stuck open
  3. An air leak or a worn compressor Correct answer.
  4. A relay valve with too much lubrication

Explanation

Slow pressure buildup or failure to reach cut-out usually points to an air leak somewhere in the system or a compressor that is not pumping efficiently. The driver should check for leaks and have the compressor inspected.

Question 11Air Brake System Components and Operation

What is the purpose of the tractor protection valve?

  1. To drain the trailer reservoirs during a pre-trip inspection
  2. To release the trailer parking brakes automatically
  3. To increase air pressure to the trailer during hard braking
  4. To close the tractor's air supply to the trailer if pressure drops or the trailer breaks away Correct answer.

Explanation

The tractor protection valve protects the tractor's air supply. If trailer pressure drops or the trailer separates, it closes off and prevents the tractor from losing all its air.

Question 12Air Brake System Components and Operation

A driver pulls the trailer air supply (red) knob out while the trailer is connected. What should happen?

  1. The compressor begins to build pressure faster
  2. Only the tractor parking brakes apply
  3. The trailer service brakes release and the tractor brakes apply
  4. The trailer emergency brakes apply and the trailer air supply is shut off Correct answer.

Explanation

Pulling the red trailer air supply knob closes the tractor protection valve and applies the trailer emergency (spring) brakes. This is the normal way to secure the trailer when uncoupling.

Question 13Air Brake System Components and Operation

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

  1. It measures the amount of air used during a leak test
  2. It prevents the parking brake from releasing until pressure is high
  3. It senses supply pressure and controls the compressor
  4. It reduces the time lag by using reservoir air close to the brake chambers to apply them quickly Correct answer.

Explanation

A relay valve is mounted near the brake chambers it serves and uses local reservoir air to apply the brakes quickly, reducing the lag that would occur if air had to travel from the foot valve. This shortens stopping distance.

Question 14Air Brake System Components and Operation

What does a spring brake (emergency/parking brake) do when air pressure is lost?

  1. It applies because the spring expands when air pressure is removed Correct answer.
  2. It applies only if the driver pulls the yellow knob
  3. It remains in whatever position it was in
  4. It releases because the spring is compressed by air

Explanation

Spring brakes are held off by air pressure. When pressure is lost, the spring expands and applies the brake automatically, providing emergency and parking braking.

Question 15Air Brake System Components and Operation

How should a driver test the spring brakes during a pre-trip inspection?

  1. Build full pressure, release the parking brake, and check that the vehicle rolls freely
  2. Drain all air and check that the parking brake releases
  3. Apply the foot valve and listen for a buzzer
  4. Build pressure, then pull the yellow parking knob and check that the vehicle does not move Correct answer.

Explanation

With normal pressure built, the driver pulls the yellow parking control knob and tries to move; the vehicle should not move if the spring brakes are working. This confirms the parking brake applies with air pressure available.

Question 16Air Brake System Components and Operation

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

  1. To reduce the pressure sent to the trailer
  2. To increase the volume of air the compressor delivers
  3. To lubricate the compressor bearings
  4. To remove moisture and contaminants from the compressed air Correct answer.

Explanation

The air dryer removes moisture and oil from compressed air before it reaches the reservoirs, helping prevent corrosion and freeze-ups. It does not increase compressor output or lubricate the compressor.

Question 17Air Brake System Components and Operation

A driver applies the brakes and hears a steady hiss that continues while the pedal is held. What does this most likely indicate?

  1. The safety valve opening during a normal stop
  2. Normal air flow through the relay valve
  3. A leak in the service brake system Correct answer.
  4. The compressor cut-out point being reached

Explanation

A continuous hiss while the brake pedal is held usually means air is escaping from the service brake system, such as a leaking chamber or line. Normal relay valve flow is brief, not steady.

Question 18Air Brake System Components and Operation

What does the air compressor governor do when pressure falls to the cut-in setting?

  1. It sounds the low-pressure warning
  2. It engages the compressor to build pressure again Correct answer.
  3. It applies the spring brakes
  4. It opens the safety valve

Explanation

When supply pressure drops to the cut-in setting, the governor allows the compressor to pump again until cut-out pressure is reached. This cycling keeps the system within its normal operating range.

Question 19Air Brake System Components and Operation

What is the function of a quick-release valve?

  1. It rapidly drains the wet tank after shutdown
  2. It releases the trailer supply when the trailer breaks away
  3. It quickly applies the parking brake when the engine stalls
  4. It speeds up the release of air from the brake chambers when the pedal is released Correct answer.

Explanation

A quick-release valve exhausts air from the brake chambers close to the axle, so the brakes release faster than if air had to travel back to the foot valve. This improves brake response and reduces drag.

Question 20Air Brake System Components and Operation

During a static leak test, the driver applies the brakes and holds them. What is the driver checking for?

  1. That the warning buzzer sounds
  2. That the trailer supply knob pops out
  3. That pressure does not drop faster than the allowed rate, indicating no significant leak Correct answer.
  4. That the compressor reaches cut-out quickly

Explanation

A static leak test checks that system pressure holds within the allowed loss rate with the brakes applied. A faster drop indicates a leak that must be repaired before driving.

Question 21Air Brake System Components and Operation

Why does a dual air brake system include check valves between the supply tank and the dry tanks?

  1. To allow air to flow only from the supply tank toward the dry tanks and prevent backflow Correct answer.
  2. To automatically drain moisture from the dry tanks
  3. To increase the pressure delivered to the brake chambers
  4. To connect the two circuits so a failure in one drains the other

Explanation

Check valves allow air to flow one way from the supply tank into each dry tank and prevent backflow. This keeps a failure in one circuit from draining the other, preserving braking capability.

Question 22Air Brake System Components and Operation

A driver performs the tractor protection valve test and the trailer air supply knob does not pop out when the pressure is reduced. What is the correct interpretation?

  1. The compressor is overcharging the system
  2. The tractor protection system may be faulty and should be checked by a mechanic Correct answer.
  3. The test is passed; the knob should stay in at all times
  4. The trailer is not connected and the test is not applicable

Explanation

During the tractor protection test, reducing pressure should cause the trailer supply knob to pop out, applying the trailer emergency brakes. If it does not, the protection system may be faulty and the vehicle should not be placed in service until repaired.

Question 23Air Brake System Components and Operation

Why is it important that the compressor's cut-out pressure be higher than the spring brake release pressure?

  1. So the trailer supply knob stays out permanently
  2. So the warning buzzer stays on during normal driving
  3. So the safety valve opens during every stop
  4. So the spring brakes can be released and the system can build enough pressure to operate the service brakes Correct answer.

Explanation

The spring brakes require a minimum pressure to release, and the system must build above that before the vehicle can move. Cut-out pressure is set high enough to release the springs and provide full service braking.

Question 24Air Brake System Components and Operation

What happens in a dual air brake system if the front circuit loses pressure?

  1. The compressor automatically switches to the front circuit
  2. The spring brakes release and the vehicle rolls freely
  3. The rear circuit can still provide braking, and a warning indicates the failure Correct answer.
  4. All braking is lost immediately

Explanation

The two circuits are isolated by check valves, so a failure in one leaves the other able to brake the vehicle. A warning alerts the driver, who should stop and have the system repaired.

Question 25Air Brake System Components and Operation

A driver notices the air pressure gauge fluctuates rapidly between the cut-in and cut-out points while driving. Which condition is the most likely explanation?

  1. The trailer supply valve left in the open position
  2. A normal governor cycle that occurs every few seconds
  3. A governor or compressor problem causing excessively frequent cycling Correct answer.
  4. The safety valve relieving pressure continuously

Explanation

A governor normally cycles the compressor only occasionally. Rapid, repeated cycling between cut-in and cut-out suggests a governor or compressor fault, or a leak, and should be inspected.

Question 26Air Brake System Components and Operation

On a typical CDL air brake system, what does the safety valve mounted on the air tank do?

  1. It closes the supply line whenever the trailer separates.
  2. It releases air if tank pressure rises above a preset maximum. Correct answer.
  3. It drains accumulated moisture every time the compressor cycles.
  4. It limits brake application pressure to the rear axle only.

Explanation

The safety valve is a pressure-relief device that vents excess air to atmosphere if the governor or compressor fails to stop the build-up at cut-out. It protects the tank and lines from over-pressurization. It is not a drain, a shutoff, or an application-pressure limiter.

Question 27Air Brake System Components and Operation

Which gauge reading tells a driver the pressure available in the primary (rear brake) circuit?

  1. The primary air pressure gauge. Correct answer.
  2. The application pressure gauge.
  3. The secondary air pressure gauge.
  4. The air filter restriction gauge.

Explanation

Dual systems have separate gauges for the primary and secondary circuits so the driver can see each circuit's stored pressure. The primary gauge reflects the circuit that typically supplies the rear brakes. The application gauge shows the pressure being delivered during a brake application, not stored supply pressure.

Question 28Air Brake System Components and Operation

What is the function of the drain cock at the bottom of an air tank?

  1. To route air to the trailer service line.
  2. To release spring brake air when parking.
  3. To set the compressor cut-out pressure.
  4. To bleed off moisture and oil that collect in the tank. Correct answer.

Explanation

Compressed air carries water vapor and oil mist that condense inside tanks, and the drain cock at the low point lets the driver expel that accumulation. Draining tanks daily helps prevent corrosion and freeze-ups. It has no role in spring brake release, governor settings, or trailer service delivery.

Question 29Air Brake System Components and Operation

Where is the air compressor on most heavy trucks driven from?

  1. An electric motor controlled by the governor.
  2. A hydraulic pump on the transmission.
  3. The trailer axle through a slip shaft.
  4. The engine, through a belt or gear drive. Correct answer.

Explanation

The compressor is engine-driven, usually by a belt or gear train, so it only pumps while the engine runs. It compresses atmospheric air and sends it to the system through the governor-controlled unloader. It is not electrically or hydraulically driven, and it is not powered by the trailer.

Question 30Air Brake System Components and Operation

What does the supply (wet) tank in an air brake system do?

  1. Holds only the air needed to release the spring brakes.
  2. Stores air that has already passed through the foot valve.
  3. Receives compressor air first and lets moisture and oil drop out. Correct answer.
  4. Supplies air directly to the trailer service gladhand.

Explanation

The supply or wet tank is the first tank after the compressor, so it collects the hottest, dirtiest air and gives moisture and oil a chance to condense and be drained. Air then passes through check valves to the dry tanks that feed the brake circuits. It does not meter air to the brakes or release spring brakes directly.

Question 31Air Brake System Components and Operation

What does the cut-out pressure setting of the governor represent?

  1. The pressure at which the low-pressure warning activates.
  2. The pressure at which the compressor stops pumping into the system. Correct answer.
  3. The pressure at which the spring brakes automatically apply.
  4. The pressure at which the safety valve opens.

Explanation

When tank pressure reaches cut-out, the governor signals the compressor's unloader to stop pumping until pressure falls to cut-in. Typical cut-out is around 120 to 125 psi. The warning device and spring brake application occur at much lower pressures, and the safety valve opens above normal operating pressure.

Question 32Air Brake System Components and Operation

In a dual air brake system, what does the foot valve do when the driver pushes the treadle?

  1. It closes the trailer supply line.
  2. It unloads the compressor and stops the air build-up.
  3. It releases the spring brakes on the drive axles.
  4. It applies air pressure to the service brakes in proportion to pedal travel. Correct answer.

Explanation

The foot or treadle valve meters air from the supply tanks to the service brake chambers, increasing delivery as the pedal is pushed farther. Releasing the pedal lets the air exhaust and the brakes release. Compressor unloading, spring brake release, and trailer supply control are handled by other valves.

Question 33Air Brake System Components and Operation

A driver hears a light, steady hiss at the treadle while holding a brake application, and the gauge slowly falls. What is the most likely cause?

  1. The spring brakes are releasing.
  2. The low-pressure warning is malfunctioning.
  3. There is a leak in the service brake system. Correct answer.
  4. The compressor is building pressure normally.

Explanation

A steady hiss while the brakes are applied, with pressure dropping, points to a leak in the service side of the system, such as a chamber, hose, or valve. Normal compressor cycling is not a continuous hiss at the treadle. The driver should identify and repair the leak before operating.

Question 34Air Brake System Components and Operation

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

  1. It prevents the compressor from over-speeding the engine.
  2. It reverses the direction of trailer air flow during a breakaway.
  3. It provides a modulated spring brake application when service air is lost. Correct answer.
  4. It converts hydraulic pressure to air pressure for the brakes.

Explanation

The inversion valve senses loss of service air and allows stored air to apply the spring brakes in a controlled, modulated way rather than a sudden full application. This helps a driver make a safe stop during an emergency. It does not reverse trailer flow, convert hydraulic pressure, or govern engine speed.

Question 35Air Brake System Components and Operation

What does the front brake limiting valve (where fitted) do on a vehicle?

  1. It increases front brake pressure for better stopping.
  2. It balances the trailer service line pressure.
  3. It reduces front brake application pressure under certain conditions. Correct answer.
  4. It holds the front brakes applied for parking.

Explanation

A front brake limiting valve can reduce the air pressure delivered to the front brakes, historically to reduce the risk of front wheel lock-up on slippery surfaces. It does not increase pressure, balance trailer lines, or serve as a parking brake. Drivers should know whether their vehicle is equipped with one and how it is set.

Question 36Air Brake System Components and Operation

A driver presses the brake pedal and the low-air warning sounds immediately, even though the gauges show normal pressure. What should the driver suspect?

  1. The warning switch or its circuit is faulty. Correct answer.
  2. The compressor is overcharging the system.
  3. The spring brakes have fully applied.
  4. The trailer supply line is disconnected.

Explanation

If the gauges show normal pressure but the warning activates, the pressure-sensing switch or its wiring is likely defective rather than the air system itself. The driver should have it checked, since a false warning can mask a real low-pressure event. Normal gauges rule out overcharging and full spring brake application.

Question 37Air Brake System Components and Operation

What is the function of the double check valve in a dual air brake system?

  1. It drains moisture from both circuits at once.
  2. It lets the higher-pressure circuit supply a shared component. Correct answer.
  3. It prevents the compressor from charging the wet tank.
  4. It releases the spring brakes when the engine is off.

Explanation

A double check valve has two inlets and one outlet, and it passes air from whichever inlet has the higher pressure. This lets a component such as the trailer supply or a warning device be fed from either circuit if one fails. It does not drain tanks, block the wet tank, or release spring brakes.

Question 38Air Brake System Components and Operation

When the parking brake control is pushed in to release the spring brakes, what must be true for the brakes to release?

  1. The trailer must be disconnected.
  2. System air pressure must be sufficient to overcome the spring force. Correct answer.
  3. The engine must be at operating temperature.
  4. The foot valve must be held down.

Explanation

Spring brakes are applied by heavy springs and released by air pressure acting against them. If system pressure is too low, pushing in the control will not release the brakes. Engine temperature, trailer connection, and pedal position do not determine release.

Question 39Air Brake System Components and Operation

What does the compressor unloader mechanism do when the governor reaches cut-out?

  1. It vents all stored air to atmosphere.
  2. It applies the trailer spring brakes.
  3. It opens the tank drains automatically.
  4. It stops the compressor from pumping air into the system. Correct answer.

Explanation

At cut-out the governor signals the unloader to hold the compressor inlet valves open so the compressor stops pumping, even though the engine keeps turning it. When pressure falls to cut-in, the unloader resets and pumping resumes. It does not drain tanks, apply trailer brakes, or dump stored air.

Question 40Air Brake System Components and Operation

A driver notices that pressure in the secondary circuit drops to zero but the primary circuit holds normal pressure. What is the expected result?

  1. The primary circuit continues to provide braking on its axles. Correct answer.
  2. The compressor stops charging the primary tank.
  3. The spring brakes release automatically.
  4. The vehicle loses all braking ability immediately.

Explanation

Dual systems are designed so that a failure in one circuit leaves the other circuit with stored pressure to stop the vehicle. The driver still has braking on the axles served by the healthy circuit, though stopping distance increases. Total loss of braking does not occur, and the compressor continues to charge the intact circuit.

Question 41Air Brake System Components and Operation

Why must the trailer service line and trailer supply line be connected correctly and not swapped?

  1. Swapping them improves brake response time.
  2. Both lines carry identical signals at all times.
  3. The supply line charges the trailer; the service line signals brake application. Correct answer.
  4. The service line is only used when parking.

Explanation

The supply (emergency) line charges the trailer tanks and holds off the trailer spring brakes, while the service line carries the application signal from the foot valve. If they are crossed, the trailer will not charge or brake correctly. They do not carry identical signals, and the service line is not a parking control.

Question 42Air Brake System Components and Operation

In a dual air brake system, what does the check valve between the supply tank and a dry tank prevent?

  1. The safety valve from opening.
  2. The compressor from reaching cut-out pressure.
  3. The trailer from charging its tanks.
  4. Air from the dry tank flowing back into the supply tank. Correct answer.

Explanation

The check valve allows air to flow from the supply tank into the dry tank but blocks reverse flow. This keeps each circuit's stored air from bleeding back through the supply tank if the compressor or another circuit fails. It does not limit compressor cut-out, defeat the safety valve, or prevent trailer charging.

Question 43Air Brake System Components and Operation

A driver performs the compressor build-up check and finds pressure rises from 85 to 100 psi in about 50 seconds. What does this indicate?

  1. The build-up is slower than the typical standard and should be investigated. Correct answer.
  2. The governor cut-out is set too high.
  3. The compressor is building pressure faster than required.
  4. The safety valve is stuck open.

Explanation

A common CDL standard is that pressure should build from 85 to 100 psi within about 45 seconds at a fast idle. Taking 50 seconds is slower than that benchmark and points to a weak compressor, a leak, or a restriction. It does not indicate over-fast build-up, high cut-out, or a stuck safety valve.

Question 44Air Brake System Components and Operation

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

  1. To lubricate the brake chambers.
  2. To introduce alcohol vapor that helps prevent moisture from freezing in the lines. Correct answer.
  3. To increase compressor output at high altitude.
  4. To reduce brake noise during application.

Explanation

An alcohol evaporator adds alcohol vapor to the compressed air so that condensed moisture is less likely to freeze and block valves or lines in cold weather. It does not boost compressor output, lubricate chambers, or quiet the brakes. Proper draining and air drying remain the primary moisture controls.

Question 45Air Brake System Components and Operation

During a pre-trip inspection, a driver shuts off the engine, releases the service brakes, and watches the gauges. Pressure drops from 120 to 110 psi in one minute with the brakes released. What does this suggest?

  1. A leak in the air system that needs correction. Correct answer.
  2. The gauges are reading the application pressure.
  3. A normal drop with the engine off and brakes released.
  4. The compressor is still charging the system.

Explanation

With the engine off and service brakes released, a common CDL standard allows no more than about 3 psi loss in one minute for a single vehicle, so a 10 psi drop is excessive. It indicates an air leak that must be found and repaired. A normal system holds pressure much more tightly.

Question 46Air Brake System Components and Operation

A driver applies the brakes fully and holds them with the engine off. Pressure drops 4 psi in one minute. How should this be interpreted against typical standards?

  1. It is excessive and indicates a major leak.
  2. It shows the governor cut-in is too low.
  3. It means the spring brakes have applied.
  4. It is within the typical allowable loss for a single vehicle. Correct answer.

Explanation

With the brakes applied and the engine off, a common standard allows up to about 4 psi loss in one minute for a single vehicle, so this reading is at the limit but acceptable. A much larger drop would indicate a service-side leak. The test does not evaluate governor settings or spring brake application.

Question 47Air Brake System Components and Operation

Why is it important that the compressor's cut-out pressure be well above the pressure needed to release the spring brakes?

  1. So the safety valve opens during each cycle.
  2. So the compressor runs continuously without cycling.
  3. So the trailer can be charged without a supply line.
  4. So the spring brakes can be released and stay released during normal operation. Correct answer.

Explanation

Spring brakes need a minimum air pressure to release, and the system must maintain pressure above that threshold during normal driving. If cut-out were too close to the release pressure, ordinary use and small leaks could drop pressure enough to drag or apply the spring brakes. Normal governor cycling is expected, and a supply line is still required to charge a trailer.

Question 48Air Brake System Components and Operation

A driver reports that the spring brakes will not release even after the system reaches normal pressure and the parking control is pushed in. Which component is most likely at fault?

  1. The drain cocks are open.
  2. The compressor is overcharging the tanks.
  3. The parking control valve is not supplying air to release the springs. Correct answer.
  4. The safety valve is opening too early.

Explanation

If system pressure is normal but the spring brakes stay applied, the parking control valve or the lines feeding the spring brake chambers are likely not delivering release air. The safety valve, overcharging, and open drains would affect pressure differently. Diagnosis should trace air from the control valve to the chambers.

Question 49Air Brake System Components and Operation

During the governor cut-in/cut-out check, a driver observes the compressor cycling between about 100 and 120 psi instead of the usual 85 to 125 psi range. What is the most useful interpretation?

  1. The system is operating normally because any cycling is acceptable.
  2. The governor settings are outside typical values and should be checked. Correct answer.
  3. The gauges must be disconnected.
  4. The spring brakes are about to apply.

Explanation

Governor settings are typically around 100 psi cut-in and 120 to 125 psi cut-out, so a 100 to 120 psi cycle is close but the narrow band and low cut-out differ from the usual values and warrant inspection. Cycling itself is normal, but the set points matter for keeping pressure adequate. Spring brakes are not about to apply at 100 psi, and the gauges are not the issue.

Question 50Air Brake System Components and Operation

A truck with a dual air system loses the primary circuit while descending a grade. What is the best driver response?

  1. Pump the brake pedal rapidly to rebuild primary pressure.
  2. Continue at the same speed because the secondary circuit is unaffected.
  3. Pull the trailer air supply knob to apply the trailer brakes only.
  4. Use the remaining circuit's brakes and gearing to slow and stop safely. Correct answer.

Explanation

With one circuit failed, the driver retains braking on the circuit that still has pressure, so the correct response is to use that braking along with downshifting and engine braking to reduce speed and stop safely. Pumping the pedal will not restore a failed circuit's pressure. Relying on the trailer alone or maintaining speed ignores the reduced braking capability.