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Alabama CDL Air Brakes study materials
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Alabama CDL Air Brakes practice questions

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Question 1Air Brake System Components and Functions

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

  1. To convert air pressure into mechanical force at the wheels
  2. To reduce air pressure delivered to the brake chambers
  3. To build and maintain the supply of compressed air in the reservoirs Correct answer.
  4. To release air from the system when pressure is too high

Explanation

The compressor is the source of compressed air and keeps the storage reservoirs charged so the system has a working supply. It does not regulate delivery pressure, apply force at the wheels, or act as the main relief device.

Question 2Air Brake System Components and Functions

What component controls when the air compressor is loading (pumping) and unloading (idling)?

  1. The safety valve
  2. The relay valve
  3. The quick release valve
  4. The governor Correct answer.

Explanation

The governor senses reservoir pressure and signals the compressor to load when pressure is low and unload when the set cut-out pressure is reached. This keeps system pressure within the proper operating range.

Question 3Air Brake System Components and Functions

What is stored in the supply reservoir of an air brake system?

  1. Hydraulic fluid for the service brakes
  2. Compressed air that feeds the rest of the system Correct answer.
  3. Air that has already passed through the brake chambers
  4. Engine coolant for the compressor

Explanation

The supply reservoir holds compressed air produced by the compressor and distributes it to the various circuits. It is the main storage tank from which other components draw air.

Question 4Air Brake System Components and Functions

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

  1. To release air if system pressure becomes dangerously high Correct answer.
  2. To drain moisture from the tank
  3. To equalize pressure between the two circuits
  4. To signal the driver when pressure is low

Explanation

The safety valve is a pressure relief device that opens if reservoir pressure rises above a safe limit, protecting the system from overpressure. It is a backup to normal governor control.

Question 5Air Brake System Components and Functions

Which component does the driver press to apply the service brakes?

  1. The trailer hand valve
  2. The foot valve (treadle valve) Correct answer.
  3. The parking brake control
  4. The compressor unloader

Explanation

The foot valve, also called the treadle valve, is operated by the brake pedal and controls the amount of air delivered to the service brake chambers. Pressing it applies the service brakes in proportion to pedal travel.

Question 6Air Brake System Components and Functions

What is the purpose of a check valve in an air brake system?

  1. To measure the pressure in each circuit
  2. To allow air to flow in only one direction Correct answer.
  3. To automatically drain water from the tank
  4. To reduce air pressure to the brake chambers

Explanation

A check valve permits air to pass in one direction and blocks reverse flow, which protects circuits and keeps air from bleeding back. This helps preserve pressure in the reservoir that should stay charged.

Question 7Air Brake System Components and Functions

What does the low air pressure warning device do?

  1. It shuts off the compressor to save energy
  2. It warns the driver when system pressure drops to an unsafe level Correct answer.
  3. It releases air from the safety valve
  4. It automatically applies the trailer brakes

Explanation

The low-pressure warning, typically a buzzer and light, alerts the driver when reservoir pressure falls below a safe operating level. This gives the driver time to stop safely before braking ability is lost.

Question 8Air Brake System Components and Functions

What is the main function of a brake chamber?

  1. To store compressed air for later use
  2. To filter moisture and oil from the air supply
  3. To convert air pressure into mechanical force that applies the brakes Correct answer.
  4. To regulate compressor output pressure

Explanation

The brake chamber receives air pressure and uses it to move a pushrod, converting pneumatic pressure into mechanical force. That force is transferred through the slack adjuster to apply the brakes.

Question 9Air Brake System Components and Functions

A driver notices that releasing the brake pedal does not promptly release the brakes on a long wheelbase vehicle. Which component is designed to speed up brake release at the chambers?

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

Explanation

A quick release valve is mounted near the brake chambers and exhausts air locally when the pedal is released, speeding release. Without it, air would have to travel all the way back to the foot valve before releasing.

Question 10Air Brake System Components and Functions

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

  1. To warn the driver of low pressure
  2. To prevent the parking brake from releasing
  3. To supply and exhaust air to brake chambers using a remote reservoir, speeding response Correct answer.
  4. To reduce the compressor's duty cycle

Explanation

A relay valve uses a small control signal to open a larger supply of air from a nearby reservoir directly to the brake chambers. This shortens the distance air must travel, improving application and release timing.

Question 11Air Brake System Components and Functions

What is the function of the slack adjuster?

  1. It regulates reservoir pressure
  2. It stores air for emergency braking
  3. It converts pushrod stroke into rotary motion to apply the brake and adjusts for lining wear Correct answer.
  4. It separates oil and water from the air

Explanation

The slack adjuster is a lever on the brake camshaft that converts the linear pushrod stroke into rotation that applies the brake. Automatic slack adjusters also compensate for brake lining wear to maintain proper adjustment.

Question 12Air Brake System Components and Functions

What provides the force that applies the parking/emergency spring brakes?

  1. Compressed air pushing on a diaphragm
  2. A heavy coil spring inside the brake chamber Correct answer.
  3. Hydraulic pressure from a master cylinder
  4. A mechanical cable pulled by the driver

Explanation

Spring brakes use a large coil spring to apply the brakes when air pressure is removed, so they set automatically if air is lost. Compressed air is used to compress the spring and release the brakes during normal operation.

Question 13Air Brake System Components and Functions

What is the role of the air dryer in a compressed air system?

  1. To cool the compressor
  2. To increase reservoir pressure
  3. To apply the trailer brakes automatically
  4. To remove moisture and contaminants from the compressed air Correct answer.

Explanation

The air dryer removes water vapor and contaminants from compressed air before it reaches the reservoirs. This helps prevent corrosion and freeze-ups in the system.

Question 14Air Brake System Components and Functions

What does a pressure gauge on the dash of an air brake vehicle indicate?

  1. The pressure in the brake circuits or reservoirs Correct answer.
  2. The compressor RPM
  3. The temperature of the brake drums
  4. The amount of air flowing through the foot valve

Explanation

The dash pressure gauge shows the air pressure available in the brake system so the driver can confirm it is within the safe operating range. Dual-needle gauges show pressure in each of the two circuits.

Question 15Air Brake System Components and Functions

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

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

Explanation

A double check valve lets air from whichever circuit has the higher pressure pass through to a shared component while blocking flow back into the lower-pressure circuit. This maintains function if one circuit fails.

Question 16Air Brake System Components and Functions

Why are most heavy vehicle air brake systems split into two separate circuits?

  1. To eliminate the need for a parking brake
  2. So that a failure in one circuit still leaves braking capability in the other Correct answer.
  3. To reduce the total weight of the vehicle
  4. To allow the compressor to run at a lower speed

Explanation

Dual circuits provide redundancy so that a loss of pressure in one circuit does not leave the vehicle without brakes. The remaining circuit can still provide braking to stop safely.

Question 17Air Brake System Components and Functions

What does the application pressure gauge (if equipped) show the driver?

  1. The compressor cut-out pressure setting
  2. The pressure in the trailer's tires
  3. The pressure in the supply reservoir only
  4. The pressure being applied to the brakes at that moment Correct answer.

Explanation

An application pressure gauge indicates how much air pressure is being sent to the brake chambers during a brake application. This helps the driver judge how hard the brakes are being applied.

Question 18Air Brake System Components and Functions

What is the purpose of the alcohol evaporator or injector sometimes found in air brake systems?

  1. To increase brake chamber pressure
  2. To clean the brake drums
  3. To lubricate the slack adjusters
  4. To help prevent moisture in the air lines from freezing Correct answer.

Explanation

An alcohol evaporator introduces alcohol vapor into the air system to lower the freezing point of any moisture present. This helps prevent ice from blocking valves and lines in cold weather.

Question 19Air Brake System Components and Functions

A driver fully applies the parking brake control and hears air exhaust from the spring brake chambers. What is happening?

  1. The compressor is overloading the system
  2. The safety valve has failed
  3. Air is being supplied to release the spring brakes
  4. Air is being released so the springs can apply the brakes Correct answer.

Explanation

Applying the parking brake control exhausts air from the spring brake chambers, allowing the large springs to extend and apply the brakes. The sound of exhausting air is normal during this action.

Question 20Air Brake System Components and Functions

What does the trailer hand valve (trolley valve) do?

  1. It applies only the trailer service brakes for testing Correct answer.
  2. It releases the trailer spring brakes
  3. It applies the tractor parking brake
  4. It controls the compressor cut-in pressure

Explanation

The trailer hand valve lets the driver apply the trailer service brakes independently, which is useful for checking that the trailer brakes work. It should not be used for parking.

Question 21Air Brake System Components and Functions

In a dual-circuit system, what happens to the tractor protection valve when trailer air pressure drops too low?

  1. It applies the trailer spring brakes directly
  2. It closes to keep the tractor's air supply from being lost through the trailer Correct answer.
  3. It opens fully to increase trailer pressure
  4. It bypasses the governor

Explanation

The tractor protection valve closes when trailer supply pressure falls too low, preventing the tractor's air from escaping through a broken or disconnected trailer line. This preserves the tractor's ability to brake.

Question 22Air Brake System Components and Functions

Why is a relay valve often mounted near the rear axle rather than at the foot valve?

  1. To shorten the air path so rear brakes apply and release faster Correct answer.
  2. To eliminate the need for a quick release valve
  3. To reduce the number of brake chambers needed
  4. To allow the compressor to run continuously

Explanation

Mounting a relay valve close to the rear brake chambers shortens the distance air must travel, improving application and release timing. It uses a small control signal from the foot valve to release a large volume of stored air locally.

Question 23Air Brake System Components and Functions

A vehicle with automatic slack adjusters has a pushrod stroke that exceeds the adjustment limit. What does this most likely indicate?

  1. The safety valve is stuck open
  2. The air dryer is saturated
  3. The governor cut-out pressure is too high
  4. A mechanical problem such as a worn or damaged slack adjuster or brake component Correct answer.

Explanation

Excessive pushrod stroke on an automatic slack adjuster usually signals a mechanical fault, such as a damaged adjuster or worn brake parts, rather than a normal condition. Automatic adjusters are not meant to be manually readjusted to mask such problems.

Question 24Air Brake System Components and Functions

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

  1. It converts hydraulic pressure to air pressure
  2. It provides a modulated air signal to release spring brakes during an emergency so the vehicle can be moved Correct answer.
  3. It inverts the pressure gauge reading
  4. It reverses the direction of the compressor

Explanation

The inversion valve supplies a controlled air signal to the spring brake chambers during certain emergency conditions, allowing the spring brakes to be released so the vehicle can be moved. It helps manage spring brake operation when primary pressure is lost.

Question 25Air Brake System Components and Functions

What is the purpose of the supply line and service line in a tractor-trailer air brake connection?

  1. Both lines carry only the parking brake signal
  2. The supply line charges the trailer reservoirs, and the service line carries the brake application signal Correct answer.
  3. The supply line carries the brake signal, and the service line drains water
  4. Both lines are used only to release the trailer spring brakes

Explanation

The supply (emergency) line delivers air to charge the trailer reservoirs, while the service line carries the control signal that applies the trailer service brakes. Keeping these functions separate allows the trailer to brake and to protect its air supply.

Question 26Air Brake System Components and Functions

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

  1. It limits the maximum pressure delivered to the service brake chambers
  2. It filters moisture and oil from the compressed air before storage
  3. It controls when the compressor loads and unloads to maintain system pressure within a set range Correct answer.
  4. It converts electrical signals into air pressure for the brake lights

Explanation

The governor senses reservoir pressure and signals the compressor to load (pump) at the cut-in pressure and unload (stop pumping) at the cut-out pressure. This keeps system pressure within the designed operating range. It does not dry or filter air, and it does not directly meter pressure to the chambers.

Question 27Air Brake System Components and Functions

Which component stores compressed air so the brakes can be applied several times even if the compressor stops running?

  1. The treadle valve
  2. The service brake chambers
  3. The supply reservoir Correct answer.
  4. The safety valve

Explanation

Reservoirs store compressed air for later use, allowing a reserve of brake applications if the compressor fails or the engine stops. The chambers only convert air pressure into mechanical force at the moment of application.

Question 28Air Brake System Components and Functions

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

  1. It signals the driver when pressure falls below a safe level
  2. It prevents air from flowing back toward the compressor
  3. It automatically drains accumulated moisture each time the brakes are released
  4. It releases air to the atmosphere if reservoir pressure rises above a preset limit Correct answer.

Explanation

The safety valve is a relief device set slightly above the governor cut-out pressure. If the governor or compressor malfunctions and pressure climbs too high, the valve vents air to protect the reservoir and other components from damage.

Question 29Air Brake System Components and Functions

What does the quick-release valve do in an air brake circuit?

  1. It opens the spring brake chambers when the park control is pulled
  2. It bypasses the foot valve if the primary circuit loses pressure
  3. It speeds up the exhaust of air from the brake chambers when the brake is released Correct answer.
  4. It rapidly fills the service reservoir during the compressor's loaded cycle

Explanation

A quick-release valve is mounted close to the brake chambers it serves. When the foot valve is released, the valve vents chamber air locally instead of forcing it back through the long line to the foot valve, so the brakes release faster.

Question 30Air Brake System Components and Functions

A driver notices the compressor keeps running and the dash gauge climbs well above the normal cut-out pressure. Which component is the most likely cause?

  1. The governor has failed to unload the compressor Correct answer.
  2. The safety valve is stuck open
  3. A service brake chamber has a ruptured diaphragm
  4. The low pressure warning switch is defective

Explanation

The governor tells the compressor when to stop pumping at cut-out pressure. If it fails to unload, the compressor keeps building pressure beyond the normal range. The safety valve should then vent to prevent damage, which is why overpressure points to a governor problem.

Question 31Air Brake System Components and Functions

What is the function of the one-way check valve placed between the compressor and the first reservoir?

  1. It drains oil and water from the discharge line automatically
  2. It regulates the compressor's cut-in and cut-out pressures
  3. It keeps air from flowing back from the reservoir to the compressor when it unloads Correct answer.
  4. It limits the volume of air the compressor can deliver per minute

Explanation

When the compressor unloads, the discharge line pressure drops. The check valve prevents stored air in the reservoir from flowing backward through the compressor and out the unloader. This protects the compressor and preserves reservoir pressure.

Question 32Air Brake System Components and Functions

In a system with a supply reservoir feeding a secondary reservoir through a pressure protection valve, what does the pressure protection valve accomplish?

  1. It closes off the secondary circuit when supply pressure drops below a set value so the primary circuit keeps working Correct answer.
  2. It vents excess air from the secondary reservoir to the atmosphere
  3. It equalizes pressure between the two circuits at all times
  4. It converts the secondary circuit to a manual backup system

Explanation

A pressure protection valve isolates a secondary circuit once supply pressure falls below a preset threshold. This preserves the remaining air for the more critical circuit so the vehicle can still be stopped.

Question 33Air Brake System Components and Functions

What is the purpose of the drain cock or automatic drain valve on an air reservoir?

  1. It allows accumulated water and oil to be expelled from the bottom of the tank Correct answer.
  2. It provides a test point for checking governor cut-out pressure
  3. It releases air to the spring brake chambers during parking
  4. It bleeds off excess pressure when the compressor overcharges the system

Explanation

Moisture and oil collect at the low point of a reservoir. The drain cock or automatic drain valve removes this contamination so it does not travel into valves and brake components and cause corrosion or freezing.

Question 34Air Brake System Components and Functions

What does the brake pedal (treadle) valve do in an air brake system?

  1. It balances pressure between the tractor and trailer supply lines
  2. It stores compressed air for the service brakes
  3. It automatically applies the spring brakes when the engine stops
  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 service brake control. Its treadle position determines how much reservoir pressure is delivered to the service brake chambers, giving the driver proportional control of braking force.

Question 35Air Brake System Components and Functions

Why is a low-pressure warning device required to activate before the spring brakes apply automatically?

  1. To keep the spring brakes from ever applying in an emergency
  2. To reset the governor to a lower cut-in pressure
  3. To prevent the compressor from restarting once pressure drops
  4. To give the driver time to stop safely before the parking brakes set on their own Correct answer.

Explanation

Spring brakes apply automatically when air pressure falls to a low threshold. The warning must come on at a higher pressure so the driver has advance notice and can move the vehicle to a safe location before the brakes lock on.

Question 36Air Brake System Components and Functions

What is the role of the spring brake (parking) control valve, often called the yellow button?

  1. It controls the flow of air that holds the spring brakes off, so pushing or pulling it applies or releases the parking brakes Correct answer.
  2. It reduces reservoir pressure to the trailer during a breakaway
  3. It sets the compressor cut-out pressure for the parking circuit
  4. It meters service brake pressure to the rear chambers during normal stops

Explanation

Spring brakes are held released by air pressure. The parking control valve supplies or exhausts that air: pushing it in charges the spring brake chambers to release the brakes, and pulling it out exhausts the air so the springs apply the brakes.

Question 37Air Brake System Components and Functions

A driver hears a constant hiss of air from a brake chamber area only when the service brakes are applied, and the pressure gauge falls faster than normal. What does this most likely indicate?

  1. A properly functioning quick-release valve
  2. A leaking service brake chamber diaphragm or its plumbing Correct answer.
  3. A governor stuck in the unloaded position
  4. A safety valve opening at its set pressure

Explanation

Air escaping only during application points to a leak on the delivery side, such as a ruptured chamber diaphragm or a leaking line or fitting. Because the leak occurs only when pressure is delivered, the reservoir drains faster with each application.

Question 38Air Brake System Components and Functions

How does an air compressor unloader mechanism work?

  1. It holds the compressor inlet valves open so the compressor stops building pressure when the governor signals cut-out Correct answer.
  2. It vents the service reservoirs to atmosphere whenever the engine is idling
  3. It redirects compressed air into the air dryer for regeneration
  4. It disconnects the compressor drive belt from the engine pulley

Explanation

When the governor reaches cut-out pressure, it sends an air signal to the unloader, which holds the compressor's intake valves open. The compressor keeps turning but cannot build pressure, so system pressure stays at the cut-out setting until the governor signals cut-in.

Question 39Air Brake System Components and Functions

Why must the supply line of a tractor-trailer combination be charged before the service line can deliver air to the trailer brakes?

  1. The supply line contains the only check valve in the combination
  2. The service line is physically smaller and cannot carry air until the supply line is full
  3. The trailer protection system requires supply pressure to open the trailer's control valves and release its spring brakes Correct answer.
  4. The trailer service brakes are electrically actuated and need the supply line as a ground path

Explanation

The trailer's emergency and control functions depend on supply-line pressure. Until the supply line is charged, the trailer protection valve stays closed and the trailer spring brakes remain applied, so service-line signals cannot produce normal braking.

Question 40Air Brake System Components and Functions

A vehicle's compressor cut-out pressure is normal, but the dash gauge drops quickly to the warning level overnight with the engine off and no brakes applied. Which component group should be inspected first?

  1. The service brake chambers and their diaphragms, because they only leak when applied
  2. The low pressure warning switch and buzzer circuit, because they monitor pressure
  3. The governor and unloader, because they control cut-out pressure
  4. The check valves and drain valves in the reservoir and supply plumbing, because they hold static pressure Correct answer.

Explanation

A system that loses pressure while sitting idle has a static leak. Check valves that fail to seat, leaking drain valves, or faulty fittings in the supply plumbing allow stored air to escape even with the brakes released. Components that leak only during application would not explain an overnight loss.

Question 41Dual Air Brake System Operation

In a dual air brake system, why are there two separate air circuits?

  1. To allow the trailer to brake before the tractor
  2. To double the braking force on steep grades
  3. To provide a backup supply if one circuit fails Correct answer.
  4. To reduce the number of gauges needed in the cab

Explanation

A dual circuit splits the system so that a failure in one circuit still leaves a functioning circuit to supply braking force. This redundancy is the core safety purpose of the split design. It does not increase total braking force or change trailer timing.

Question 42Dual Air Brake System Operation

On a dual air brake system, the two circuits are commonly described as which pair?

  1. Service and emergency
  2. Supply and control
  3. Front and rear
  4. Primary and secondary Correct answer.

Explanation

The two halves of a dual air brake system are referred to as the primary and secondary circuits. Each has its own reservoir and protection valve so one can keep working if the other fails. Service and emergency describe brake functions rather than the split circuits.

Question 43Dual Air Brake System Operation

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

  1. It signals the compressor to stop running
  2. It balances pressure between the two circuits
  3. It drains moisture from the tank daily
  4. It releases air if pressure exceeds a safe limit Correct answer.

Explanation

The safety relief valve is a pressure-limiting device that vents air if reservoir pressure climbs above a safe setting. This protects tanks and components from over-pressurization. Moisture removal and compressor control are handled by other parts.

Question 44Dual Air Brake System Operation

What does the governor do in an air brake system?

  1. It controls when the compressor loads and unloads Correct answer.
  2. It releases the spring brakes during normal operation
  3. It limits the maximum pressure in the service line
  4. It meters air to the trailer relay valve

Explanation

The governor senses reservoir pressure and tells the compressor when to pump (load) and when to stop pumping (unload). This keeps system pressure within the designed operating range. It does not directly meter air to the brakes.

Question 45Dual Air Brake System Operation

When air pressure in a dual system drops below a safe level, what should the low pressure warning device do?

  1. Provide a visible and audible warning to the driver Correct answer.
  2. Open the safety relief valve
  3. Increase engine speed to build pressure faster
  4. Automatically apply the trailer brakes only

Explanation

The low pressure warning must alert the driver with both a light and a buzzer (or similar audible device) before pressure falls into an unsafe range. This gives the driver time to stop safely. It does not automatically apply only trailer brakes or change engine speed.

Question 46Dual Air Brake System Operation

At what pressure must the low pressure warning signal activate in most air brake systems?

  1. At exactly 100 psi
  2. Before pressure drops below about 60 psi Correct answer.
  3. When pressure reaches the governor cut-out
  4. Only after pressure falls below 20 psi

Explanation

The warning device must operate before reservoir pressure drops below roughly 60 psi, giving the driver time to react. Waiting until pressure is much lower leaves too little margin for safe stopping. Cut-out pressure is a normal high operating point, not a warning threshold.

Question 47Dual Air Brake System Operation

The spring brakes on a vehicle with air brakes are held off (released) by:

  1. Air pressure from the supply circuit Correct answer.
  2. Vacuum from the engine
  3. Hydraulic fluid from the master cylinder
  4. A mechanical cable and lever

Explanation

Spring brakes use heavy springs to apply the brakes, and air pressure must be present to compress the springs and release the brakes. If air pressure is lost, the springs apply the brakes automatically. This is why they are also called emergency or parking brakes.

Question 48Dual Air Brake System Operation

During normal driving with full air pressure, the spring brakes are:

  1. Held applied by mechanical linkage
  2. Applied lightly to assist service brakes
  3. Held in the released position by air pressure Correct answer.
  4. Disconnected from the system

Explanation

With normal pressure, air compresses the spring and keeps the parking/emergency brakes released so the vehicle can move. The service brakes handle normal stopping. Spring brakes engage when pressure is lost or the parking control is set.

Question 49Dual Air Brake System Operation

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

  1. To increase the compressor's output pressure
  2. To lubricate the brake shoes
  3. To cool the air before it reaches the treadle valve
  4. To remove moisture that can freeze or corrode components Correct answer.

Explanation

Moisture in compressed air can freeze in cold weather and cause corrosion, so an evaporator or dryer removes it. Clean, dry air keeps valves and lines working reliably. These devices do not boost pressure or lubricate brake shoes.

Question 50Dual Air Brake System Operation

The compressor in an air brake system is normally driven by:

  1. The rear axle through a driveshaft
  2. An electric motor powered by the battery
  3. A dedicated air turbine
  4. The engine through a belt or gear drive Correct answer.

Explanation

The air compressor is typically engine-driven by a belt or gear so it can build pressure whenever the engine runs. It pumps air into the reservoirs until the governor unloads it. It is not driven by the axle or a separate motor.