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MD CDL Air Brakes study materials
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MD CDL Air Brakes practice questions

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Question 1Air Brake System Components and Their Functions

On a CDL air brake system, what is the primary function of the air compressor?

  1. It filters moisture and oil from the air supply
  2. It releases air to the atmosphere during a brake application
  3. It converts air pressure into mechanical force at the wheel
  4. It builds and maintains air pressure in the storage reservoirs Correct answer.

Explanation

The compressor is the source of pressurized air for the whole system; it draws in atmospheric air and pumps it into the reservoirs until the governor cuts it out at the set pressure. It does not itself apply the brakes or condition the air.

Question 2Air Brake System Components and Their Functions

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

  1. It controls the rate at which air escapes during a brake release
  2. It sets the parking brake automatically when pressure falls
  3. It limits the maximum pressure the brake chambers can receive
  4. It regulates the compressor by cutting it in and out at preset pressures Correct answer.

Explanation

The governor monitors reservoir pressure and signals the compressor to stop pumping at the cut-out pressure and to resume at the cut-in pressure. This cycling keeps system pressure within the designed operating range.

Question 3Air Brake System Components and Their Functions

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

  1. It stores reserve air for emergency brake applications
  2. It lubricates the compressor during operation
  3. It removes moisture and contaminants from the compressed air before it reaches the reservoirs Correct answer.
  4. It increases the pressure of air leaving the compressor

Explanation

The air dryer is placed between the compressor and the reservoirs to remove water vapor and oil that would otherwise collect in the tanks and lines. Dry air prevents corrosion and freeze-ups in the system.

Question 4Air Brake System Components and Their Functions

When the driver presses the brake pedal, what does the foot valve (treadle valve) do?

  1. It signals the compressor to increase output
  2. It vents all reservoir pressure to the atmosphere to warn of a leak
  3. It engages the spring brakes by releasing air from the chambers
  4. It meters air pressure from the reservoirs to the brake chambers in proportion to pedal travel Correct answer.

Explanation

The foot valve is the driver's primary control; it admits air from the supply reservoirs to the delivery lines in amounts that follow pedal position. More pedal travel delivers more pressure, giving the driver proportional braking control.

Question 5Air Brake System Components and Their Functions

What component converts air pressure into the mechanical force that moves the brake shoes or pads?

  1. The safety valve
  2. The low-pressure warning switch
  3. The brake chamber Correct answer.
  4. The relay valve

Explanation

The brake chamber contains a diaphragm and pushrod; air pressure against the diaphragm pushes the rod out, and that linear force is transferred through the slack adjuster to the foundation brake. It is the point where pneumatic energy becomes mechanical work.

Question 6Air Brake System Components and Their Functions

What is the function of the slack adjuster in an air brake system?

  1. It releases air from the chamber when the pedal is released
  2. It limits air pressure to the brake chamber
  3. It converts the pushrod stroke into rotation of the brake camshaft and adjusts for lining wear Correct answer.
  4. It stores air for emergency braking

Explanation

The slack adjuster is a lever arm mounted on the camshaft; the pushrod pushes one end, turning the camshaft and applying the brake. Automatic slack adjusters also maintain correct pushrod travel as linings wear.

Question 7Air Brake System Components and Their Functions

What is the purpose of the low-pressure warning device in a CDL air brake system?

  1. It alerts the driver when system pressure drops below a safe operating level Correct answer.
  2. It prevents the compressor from over-pressurizing the reservoirs
  3. It drains water from the reservoirs automatically
  4. It automatically applies the spring brakes when pressure is low

Explanation

The warning device, usually a buzzer and light, activates when reservoir pressure falls below a threshold so the driver can stop safely before braking performance is lost. It is an alert, not a control that applies the brakes.

Question 8Air Brake System Components and Their Functions

Which component stores the compressed air that the brake system uses for repeated applications?

  1. The supply reservoir Correct answer.
  2. The slack adjuster
  3. The brake chamber
  4. The air compressor

Explanation

Supply reservoirs hold compressed air so the brakes can be applied several times even if the compressor stops running. This reserve capacity is essential for safe stopping and for the system's emergency features.

Question 9Air Brake System Components and Their Functions

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

  1. It reduces pressure to the trailer brakes when the trailer is empty
  2. It uses a small control signal to deliver reservoir air quickly to the trailer brake chambers Correct answer.
  3. It charges the trailer spring brakes during parking
  4. It controls the compressor cut-in and cut-out pressures

Explanation

A relay valve allows the small control pressure from the foot valve to open a larger supply of reservoir air close to the trailer brakes. This shortens the distance air must travel and speeds up brake application and release on long combinations.

Question 10Air Brake System Components and Their Functions

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

  1. It drains moisture from the reservoir
  2. It exhausts air from the brake chambers rapidly to speed up brake release Correct answer.
  3. It applies the brakes faster by boosting supply pressure
  4. It isolates a leaking circuit from the rest of the system

Explanation

A quick-release valve is mounted near the brake chambers and vents chamber air to the atmosphere locally when the control pressure is released. This shortens release time that would otherwise be slowed by long lines back to the foot valve.

Question 11Air Brake System Components and Their Functions

In a dual-circuit air brake system, what is the purpose of the double check valve?

  1. It doubles the air pressure delivered to the brake chambers
  2. It releases air from both circuits at the same time
  3. It monitors the pressure in both circuits and warns the driver
  4. It allows air from either circuit to supply a common component while preventing backflow into the other circuit Correct answer.

Explanation

The double check valve lets the higher-pressure source pass through while blocking reverse flow, so a shared component can be fed by either circuit. This preserves braking capability if one circuit fails.

Question 12Air Brake System Components and Their Functions

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

  1. It drains water from the bottom of the tank
  2. It warns the driver when pressure is low
  3. It releases air to the atmosphere if reservoir pressure exceeds a preset limit Correct answer.
  4. It prevents air from flowing back to the compressor

Explanation

The safety valve is a spring-loaded relief device that opens at a predetermined pressure to protect the reservoir and lines from over-pressurization. If the governor fails to cut out the compressor, this valve limits the pressure.

Question 13Air Brake System Components and Their Functions

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

  1. It stores air for the service brake circuit
  2. It applies the brake mechanically by spring force when air pressure is reduced Correct answer.
  3. It regulates the pressure delivered to the service brake chamber
  4. It applies the brake by air pressure only during normal service braking

Explanation

The spring brake chamber contains a large spring that is held compressed by air pressure; when pressure drops, the spring extends and applies the brake. This provides automatic emergency braking and a parking brake without air.

Question 14Air Brake System Components and Their Functions

What does the compressor discharge line deliver, and where does it go?

  1. Air from the governor to the warning buzzer
  2. Air from the brake chambers to the atmosphere
  3. Hot, high-pressure air from the compressor to the air dryer and then to the reservoirs Correct answer.
  4. Cooled air from the reservoirs back to the compressor

Explanation

The discharge line carries compressed air from the compressor outlet to the air dryer, which removes moisture, and then on to the supply reservoirs. This line is under high pressure and high temperature.

Question 15Air Brake System Components and Their Functions

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

  1. It lubricates the brake chambers
  2. It cleans carbon deposits from the compressor
  3. It increases the octane rating of the fuel
  4. It introduces alcohol vapor into the air to help prevent moisture in the lines from freezing Correct answer.

Explanation

Alcohol evaporators add alcohol vapor to the compressed air so that any water in the lines and valves is less likely to freeze in cold weather. This protects brake valves and lines from ice blockage.

Question 16Air Brake System Components and Their Functions

What is the function of the pressure protection valve in a multi-circuit air brake system?

  1. It vents excess pressure to the atmosphere
  2. It reduces pressure to the trailer brakes when the trailer is unladen
  3. It isolates a failed circuit so the remaining circuits can maintain pressure Correct answer.
  4. It controls the compressor's cut-out pressure

Explanation

The pressure protection valve closes off a leaking or failed circuit once pressure falls below a set value, preserving air in the healthy circuits. This allows the vehicle to retain braking capability despite a leak.

Question 17Air Brake System Components and Their Functions

What is the purpose of a tractor protection valve?

  1. It regulates the compressor output
  2. It protects the tractor air supply by closing the trailer lines if trailer pressure drops or the trailer separates Correct answer.
  3. It prevents the tractor's spring brakes from releasing
  4. It limits the pressure delivered to the tractor's front brakes

Explanation

The tractor protection valve senses trailer supply pressure; if it falls too low or the lines separate, the valve closes to keep the tractor's air from escaping through the trailer lines. This preserves tractor braking and helps apply the trailer spring brakes.

Question 18Air Brake System Components and Their Functions

A driver notices the low-pressure warning buzzer sounds and the spring brakes apply while driving. Which component is responsible for applying the brakes in this situation?

  1. The governor, because it cut the compressor out too early
  2. The spring brake chambers, because loss of air pressure releases the holding spring Correct answer.
  3. The relay valve, because it boosted pressure too high
  4. The service brake chambers, because the foot valve lost its supply

Explanation

Spring brakes are held off by air pressure; when system pressure drops below the spring's release threshold, the spring extends and applies the brake automatically. This is the emergency braking feature of the system.

Question 19Air Brake System Components and Their Functions

During a pre-trip inspection, the driver hears the compressor cut out at about 120 psi and cut in again at about 100 psi. What does this indicate about the governor?

  1. The safety valve is opening prematurely
  2. The governor is defective because the compressor should never cut out
  3. The governor is cycling the compressor within the normal cut-out and cut-in range Correct answer.
  4. The air dryer is bypassing the reservoirs

Explanation

The governor is designed to stop the compressor at a set cut-out pressure and restart it at a lower cut-in pressure, typically around 100 to 120 psi. Hearing this cycling is a sign the governor is working correctly.

Question 20Air Brake System Components and Their Functions

A driver releases the parking brake but the vehicle will not move because the spring brakes seem to stay applied. Which component is most directly involved in releasing the spring brakes?

  1. The safety valve, which must be opened
  2. The low-pressure warning switch, which must be reset
  3. The slack adjusters, which must be backed off manually
  4. The spring brake chambers, which must receive air pressure to compress the spring Correct answer.

Explanation

Spring brakes are released by admitting air pressure to the spring brake chamber, which compresses the spring and frees the brake. If supply pressure is insufficient, the spring stays applied and the vehicle will not roll.

Question 21Air Brake System Components and Their Functions

When the foot valve is released, how does air leave the service brake chambers in a typical system?

  1. It returns to the supply reservoir for reuse
  2. It is pumped back into the compressor intake
  3. It exhausts through the foot valve or a quick-release valve to the atmosphere Correct answer.
  4. It is absorbed by the air dryer

Explanation

On release, the foot valve or a quick-release valve opens an exhaust port so chamber air escapes to the atmosphere. This drop in chamber pressure lets the diaphragm and pushrod return, releasing the brake.

Question 22Air Brake System Components and Their Functions

A driver performs a static leak test and finds that pressure in one circuit drops rapidly while the other holds steady. Which component is designed to keep the failure from draining the entire system?

  1. The pressure protection valve Correct answer.
  2. The low-pressure warning switch
  3. The slack adjuster
  4. The air dryer

Explanation

Pressure protection valves isolate a leaking circuit once pressure falls below their setting, allowing the remaining circuit to hold air. This preserves partial braking capability and is why dual-circuit systems are required.

Question 23Air Brake System Components and Their Functions

Why does a relay valve improve brake response on a long combination vehicle compared with routing all control air through the foot valve?

  1. It eliminates the need for spring brakes
  2. It bypasses the air dryer to increase flow
  3. It raises the system pressure above the governor cut-out
  4. It uses reservoir air near the brakes so the chambers fill faster than air traveling the full line length Correct answer.

Explanation

A relay valve is mounted close to the brakes it serves and opens a local supply of reservoir air when it receives a small control signal. Because the air does not have to travel from the foot valve, application and release are quicker and more uniform across the combination.

Question 24Air Brake System Components and Their Functions

A driver reports that the trailer brakes apply and release more slowly than the tractor brakes. Which component should be inspected first for a restriction or fault?

  1. The compressor discharge valve
  2. The tractor's low-pressure warning switch
  3. The trailer relay valve and its supply line Correct answer.
  4. The alcohol evaporator

Explanation

Trailer brake timing depends on the relay valve and the supply line that feeds it; a restricted or faulty relay valve slows application and release of the trailer brakes. The tractor brakes are unaffected, which points to the trailer-side control and supply components.

Question 25Air Brake System Components and Their Functions

Which air brake component is driven by the engine and supplies compressed air to the reservoirs?

  1. The governor
  2. The air compressor Correct answer.
  3. The foot valve
  4. The inversion valve

Explanation

The air compressor is engine-driven and is the source of all compressed air in the system, pumping air into the reservoirs until the governor signals it to stop. The governor only controls when the compressor loads and unloads; it does not produce air itself.

Question 26Air Brake System Components and Their Functions

What is the function of the air compressor governor?

  1. It regulates the compressor's cut-in and cut-out pressure points Correct answer.
  2. It controls the rate at which spring brakes release
  3. It filters moisture and oil out of the compressed air
  4. It limits maximum pressure in the service brake chambers

Explanation

The governor monitors 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 the designed operating range.

Question 27Air Brake System Components and Their Functions

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

  1. It removes moisture and contaminants from the compressed air before it reaches the reservoirs Correct answer.
  2. It automatically drains the reservoirs each time the brakes are applied
  3. It boosts system pressure when the compressor is unloaded
  4. It supplies air directly to the service brake chambers

Explanation

The air dryer uses a desiccant to remove water vapor and oil from compressed air before it enters the wet tank and downstream reservoirs. This reduces corrosion and prevents moisture from freezing in the lines.

Question 28Air Brake System Components and Their Functions

What is the function of the treadle (foot) valve in a CDL air brake system?

  1. It stores air for the trailer brakes
  2. It meters air pressure to the brake chambers in proportion to how far the pedal is pushed Correct answer.
  3. It signals the compressor to cut out at maximum pressure
  4. It releases spring brake air when the parking brake is set

Explanation

The foot valve is the driver's primary control: pressing the treadle admits air from the reservoirs to the brake chambers in proportion to pedal travel, and releasing it exhausts that air. This gives the driver progressive control of braking force.

Question 29Air Brake System Components and Their Functions

What is the purpose of the supply (wet) reservoir in a CDL air brake system?

  1. It is the first tank to receive compressor air and collects moisture before air moves to other circuits Correct answer.
  2. It holds air for the parking brake only
  3. It supplies air only to the trailer brakes
  4. It stores air for the low-pressure warning buzzer

Explanation

The supply or wet reservoir receives air directly from the compressor and dryer, so it is where most condensation collects. Air then flows to the primary and secondary circuits, helping keep moisture out of the brake valves.

Question 30Air Brake System Components and Their Functions

Which valve controls the flow of air from the tractor to the trailer and protects the tractor if the trailer separates?

  1. The governor
  2. The tractor protection valve Correct answer.
  3. The quick-release valve
  4. The alcohol evaporator

Explanation

The tractor protection valve closes the tractor-to-trailer air lines when pressure drops or the trailer is disconnected, preserving the tractor's air supply. It also prevents a massive air loss if the trailer breaks away.

Question 31Air Brake System Components and Their Functions

What is the function of the inversion valve in a vehicle with spring brakes?

  1. It reverses the direction of air flow in the supply line
  2. It allows the spring brakes to be released or modulated when service air pressure is lost Correct answer.
  3. It inverts the trailer's relay valve timing
  4. It converts electrical signals into air pressure

Explanation

The inversion valve provides a way to release or control spring brakes in an emergency when normal service air is unavailable, often working with the parking brake valve. It helps ensure the vehicle can still be moved or modulated in a pressure-loss situation.

Question 32Air Brake System Components and Their Functions

What is the function of the brake chamber pushrod and slack adjuster assembly?

  1. It regulates compressor cut-out pressure
  2. It converts the chamber's linear push into rotary motion that applies the brake shoes Correct answer.
  3. It stores air for the spring brake
  4. It filters moisture from the air lines

Explanation

The pushrod moves outward when air enters the chamber, and the slack adjuster converts that straight-line motion into rotation of the S-cam or camshaft to apply the brakes. It also maintains the correct shoe-to-drum clearance.

Question 33Air Brake System Components and Their Functions

What is the purpose of the low-pressure warning switch in a CDL air brake system?

  1. It signals the compressor to load
  2. It automatically applies the parking brake
  3. It activates a buzzer or light when system pressure falls below a safe level Correct answer.
  4. It limits pressure in the service chambers

Explanation

The low-pressure warning switch senses reservoir pressure and triggers a visible and audible warning when pressure drops below the safe threshold. This alerts the driver before braking performance is compromised.

Question 34Air Brake System Components and Their Functions

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

  1. It removes alcohol from the air supply
  2. It adds alcohol vapor to the air to prevent moisture from freezing in valves and lines Correct answer.
  3. It cools the compressor discharge air
  4. It evaporates fuel to help start the engine

Explanation

The alcohol evaporator introduces alcohol vapor into the air system so that any moisture present is less likely to freeze in valves, lines, or the dryer. This is especially useful in cold weather.

Question 35Air Brake System Components and Their Functions

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

  1. It prevents the compressor from overloading
  2. It allows the higher-pressure circuit to supply a shared component while blocking backflow to the other circuit Correct answer.
  3. It doubles the pressure delivered to the brake chambers
  4. It checks the system twice per second for leaks

Explanation

The double check valve compares pressure from two circuits and lets the higher pressure pass through to a shared component, such as the trailer supply or a warning device. It prevents air from one circuit from flowing backward into the other.

Question 36Air Brake System Components and Their Functions

Which component is designed to release air from a brake chamber quickly when the brake pedal is released, improving release response?

  1. The alcohol evaporator
  2. The safety valve
  3. The governor
  4. The quick-release valve Correct answer.

Explanation

A quick-release valve is mounted near the brake chambers and exhausts air locally when the control pressure drops, rather than forcing air back through a long line to the foot valve. This speeds brake release, especially on long wheelbases and trailers.

Question 37Air Brake System Components and Their Functions

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

  1. It allows the driver to drain moisture manually
  2. It vents excess pressure to protect the reservoir from overpressure Correct answer.
  3. It signals the compressor to load
  4. It prevents air from flowing to the trailer

Explanation

The safety valve is a pressure-relief device that opens if reservoir pressure exceeds a set limit, protecting the tank and lines from damage. It is a backup safeguard in case the governor fails.

Question 38Air Brake System Components and Their Functions

A driver notices the compressor runs continuously and system pressure climbs well above the normal cut-out point. Which component is most likely faulty?

  1. The low-pressure warning switch
  2. The governor Correct answer.
  3. The slack adjuster
  4. The quick-release valve

Explanation

The governor is responsible for unloading the compressor at the cut-out pressure. If it fails to signal unload, the compressor keeps pumping and pressure rises past the normal range, which is why the safety valve may eventually vent.

Question 39Air Brake System Components and Their Functions

During a pre-trip inspection, a driver finds that air pressure builds very slowly and the compressor never reaches cut-out pressure. Which component should be inspected first?

  1. The trailer relay valve
  2. The parking brake valve
  3. The compressor and its drive belt Correct answer.
  4. The quick-release valve

Explanation

Slow pressure buildup points to the air source: a slipping or broken compressor belt, a faulty compressor, or a major leak on the supply side. The trailer relay, parking valve, and quick-release valve affect application and release, not the rate at which the compressor builds pressure.

Question 40Air Brake System Components and Their Functions

A driver reports that the spring brakes will not release even though system pressure is normal and the parking brake valve is in the release position. Which component is the most likely cause?

  1. The inversion valve or its supply circuit Correct answer.
  2. The low-pressure warning switch
  3. The compressor discharge line
  4. The air dryer desiccant

Explanation

Spring brakes release when air is delivered to the spring brake chambers through the parking brake valve and, on some vehicles, the inversion valve. If that valve or its supply circuit fails, the spring brakes stay applied even with normal system pressure.

Question 41Air Supply, Compressor, and Reservoirs

On a CDL air brake system, what is the primary function of the compressor?

  1. To filter moisture and oil out of the air supply
  2. To reduce air pressure before it reaches the brake chambers
  3. To build and maintain air pressure in the storage reservoirs Correct answer.
  4. To release spring brakes when pressure drops

Explanation

The compressor is the source of compressed air for the entire system; it builds pressure that is stored in the reservoirs for later use by the brakes. Moisture removal is handled by the air dryer, and pressure protection is handled by valves, not the compressor itself.

Question 42Air Supply, Compressor, and Reservoirs

A typical CDL vehicle air system is designed to build from 85 psi to 100 psi within what time frame during the compressor check?

  1. About 3 minutes
  2. About 45 seconds Correct answer.
  3. About 1 minute
  4. About 10 minutes

Explanation

The standard air brake pre-trip check requires the system to build from 85 to 100 psi in roughly 45 seconds at normal governed engine speed. A much slower build indicates a weak compressor, a leak, or a restricted intake.

Question 43Air Supply, Compressor, and Reservoirs

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

  1. To cool the compressor during heavy use
  2. To increase the pressure delivered to the brake chambers
  3. To automatically apply the parking brake when pressure is low
  4. To remove moisture and contaminants from compressed air before it enters the reservoirs Correct answer.

Explanation

The air dryer uses a desiccant and often a purge cycle to strip water vapor and oil from compressed air so it does not collect in the reservoirs and freeze or corrode the system. It does not boost pressure or control the parking brake.

Question 44Air Supply, Compressor, and Reservoirs

The governor on an air compressor serves which function?

  1. It drains accumulated water from the wet tank
  2. It cycles the compressor between loading and unloading to control system pressure Correct answer.
  3. It limits vehicle speed when air pressure is low
  4. It meters air flow to each individual brake chamber

Explanation

The governor senses reservoir pressure and tells the compressor when to pump (loaded) and when to stop pumping (unloaded), holding system pressure within the designed cut-in and cut-out range. It does not distribute air to chambers or drain tanks.

Question 45Air Supply, Compressor, and Reservoirs

What is the wet tank in a multi-tank air brake system?

  1. The tank used to store air for the horn and wipers
  2. The tank that supplies air only to the parking brake circuit
  3. The small tank mounted at the rear axle for the trailer brakes
  4. The first reservoir that receives air directly from the compressor and collects moisture Correct answer.

Explanation

The wet tank (supply tank) is the first reservoir downstream of the compressor and dryer, so it collects most of the moisture and contaminants before air moves to the dry tanks. It feeds the rest of the system rather than serving a single component.

Question 46Air Supply, Compressor, and Reservoirs

What do the cut-in and cut-out pressures of a typical air brake system describe?

  1. The pressures at which the spring brakes apply and release
  2. The minimum and maximum readings on the dash gauge face
  3. The pressures at which the low-pressure warning buzzer sounds
  4. The pressures at which the governor loads and unloads the compressor Correct answer.

Explanation

Cut-out is the upper pressure at which the governor unloads the compressor, and cut-in is the lower pressure at which it loads again to rebuild pressure. These settings keep reservoir pressure within a safe operating band.

Question 47Air Supply, Compressor, and Reservoirs

Which component protects the primary and secondary circuits from losing pressure to a failed circuit?

  1. The pressure protection valve Correct answer.
  2. The quick release valve
  3. The inversion valve
  4. The slack adjuster

Explanation

Pressure protection valves isolate a failed or leaking circuit so the remaining circuits can still build and hold enough pressure to stop the vehicle. They close off flow to a circuit that drops below a set pressure.

Question 48Air Supply, Compressor, and Reservoirs

During the pre-trip air leak test, the system should lose no more than what amount of pressure in one minute with the engine off and brakes released?

  1. 20 psi
  2. 10 psi
  3. 1 psi
  4. 3 psi Correct answer.

Explanation

With a fully charged system, engine off, and brakes released, a leak rate above about 3 psi per minute indicates a problem that should be repaired before driving. With the brakes applied, a somewhat higher allowance (commonly about 4 psi per minute) applies.

Question 49Air Supply, Compressor, and Reservoirs

A driver notices the compressor takes much longer than normal to build pressure from 85 to 100 psi. Which condition best explains this?

  1. An air dryer purge valve stuck closed
  2. A slack adjuster that is out of adjustment
  3. A governor that is stuck in the unloaded position only
  4. A restricted compressor intake or a worn compressor Correct answer.

Explanation

Slow pressure buildup points to reduced compressor output, often from a clogged intake filter, leaking discharge line, or worn compressor components. Governor and dryer faults can also affect cycling, but a restricted intake is the classic cause of a slow build.

Question 50Air Supply, Compressor, and Reservoirs

The dash air pressure gauge shows pressure dropping steadily while the vehicle is parked with the engine off and the brakes released. What is the most likely cause?

  1. A compressor that is building too much pressure
  2. A governor stuck in the loaded position
  3. An air dryer that is purging too often
  4. An air leak somewhere in the system Correct answer.

Explanation

A steady pressure loss with the engine off and brakes released means air is escaping through a leak in a line, fitting, valve, or tank. The governor and compressor are not active contributors when the engine is off and pressure is not being generated.