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CDL Air Brakes practice questions

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Question 1Air Brake System Components and How They Work

What is the job of the air compressor in an air brake system?

  1. It releases air when tank pressure gets too high.
  2. It creates the hydraulic pressure for the brakes.
  3. It removes moisture from the brake lines.
  4. It pumps air into the storage tanks. Correct answer.

Explanation

The compressor is the source of compressed air for the whole system; it runs while the engine runs and charges the storage tanks. From the tanks, air travels to the valves and brake chambers that apply the brakes. Moisture removal is handled by drains and an air dryer, not the compressor.

Question 2Air Brake System Components and How They Work

In most commercial vehicles, what drives the air compressor?

  1. A vacuum pump driven by exhaust flow
  2. An electric motor mounted on the frame
  3. The engine, through a belt or gear Correct answer.
  4. The rear axle through a drive shaft

Explanation

The compressor is mounted on the engine and is turned by a belt or by gears driven from the engine. That is why the system builds air only when the engine is running. There is no separate electric or axle-driven compressor in a typical air brake system.

Question 3Air Brake System Components and How They Work

What does the air pressure gauge on the dash show?

  1. Engine oil pressure
  2. Brake fluid level
  3. Temperature of the compressor
  4. Pressure in the air tanks Correct answer.

Explanation

The dash gauge is the driver's view of reservoir pressure, showing how much air is available for braking. A dual system gauge shows both circuits, usually as two needles. Oil pressure and coolant or compressor temperature are shown on separate gauges.

Question 4Air Brake System Components and How They Work

What does the safety valve on an air tank do?

  1. Adds extra air during hard braking
  2. Drains water out of the tank
  3. Warns the driver of low pressure
  4. Vents air if tank pressure gets too high Correct answer.

Explanation

The safety valve is a pressure-relief device, typically set to open around 150 psi, that protects the tank and lines if the governor fails and the compressor keeps pumping. It works only as a backup; normal pressure control belongs to the governor. Draining water is done by the drain cocks at the bottom of the tank.

Question 5Air Brake System Components and How They Work

What is the purpose of a slack adjuster?

  1. It takes up slack from lining wear Correct answer.
  2. It holds the shoes away from the drum
  3. It links the pedal to the compressor
  4. It measures air pressure in the chamber

Explanation

As brake linings wear, the pushrod has to travel farther to apply the brakes; the slack adjuster is the lever on the brake cam shaft that removes that play and keeps pushrod stroke within limits. Automatic slack adjusters do this during normal brake use. A slack adjuster has no role in making or measuring air pressure.

Question 6Air Brake System Components and How They Work

What do the brake chambers do?

  1. They turn air pressure into force Correct answer.
  2. They cool the drums during long downgrades
  3. They measure how hard the driver is pressing
  4. They store compressed air for later use

Explanation

Air pressure entering the chamber pushes a diaphragm and pushrod, converting air pressure into the mechanical force that applies the brake. Storage is the job of the tanks, and the driver's pedal effort is sensed by the foot valve. Chambers are the point where air pressure becomes physical braking force.

Question 7Air Brake System Components and How They Work

What is the purpose of the drain valves on the air tanks?

  1. To test the governor cut-out setting
  2. To send air to the brake chambers
  3. To drain water and oil from tanks Correct answer.
  4. To add air pressure to the system

Explanation

Compressed air carries moisture and oil vapor that condense in the reservoirs, and each tank has a drain cock so the driver can blow this out during the pre-trip inspection. Water left in a tank can corrode it and can freeze in lines in cold weather. Drains remove contaminants; they do not control pressure.

Question 8Air Brake System Components and How They Work

What is another name for the valve the driver pushes down to apply the brakes?

  1. The treadle (foot) valve Correct answer.
  2. The check valve
  3. The relay valve
  4. The safety valve

Explanation

The treadle valve, also called the foot valve, is operated directly by the brake pedal and controls how much air reaches the brake chambers. Relay and check valves are downstream or auxiliary components that respond to the foot valve. The safety valve protects the tank from overpressure.

Question 9Air Brake System Components and How They Work

At about what pressure does the governor cut out and stop the compressor from building more air?

  1. 175 psi
  2. 125 psi Correct answer.
  3. 45 psi
  4. 75 psi

Explanation

The governor senses reservoir pressure and unloads the compressor at the cut-out point, which is typically around 125 psi (roughly 120 to 140 psi depending on the vehicle). Cut-out is the top of the normal operating range shown on the dash gauge. The compressor resumes once pressure falls to the cut-in point.

Question 10Air Brake System Components and How They Work

When reservoir pressure falls to about the cut-in point, what does the governor do?

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

Explanation

When pressure drops to the cut-in point, usually about 100 psi, the governor lets the compressor resume pumping until the cut-out point is reached again. This cycling keeps the tanks within the normal operating range. The governor does not apply brakes or sound warnings.

Question 11Air Brake System Components and How They Work

In a typical air brake system, which tank catches moisture and oil first?

  1. The supply, or wet, tank Correct answer.
  2. The parking brake tank
  3. The air dryer reservoir
  4. The secondary (front brake) tank

Explanation

Air leaves the compressor and enters the supply tank, often called the wet tank, before it is distributed to the other reservoirs. Because it is first in line, it collects most of the moisture and oil and must be drained regularly. The other tanks receive air that has already passed through the supply tank.

Question 12Air Brake System Components and How They Work

Why do spring brakes apply when air pressure is lost?

  1. A governor opens the brake chambers
  2. Air pressure pushes the shoes out
  3. A strong spring pushes them on Correct answer.
  4. The foot valve closes automatically

Explanation

Spring brakes are held in the released position by air pressure compressing a heavy spring; when pressure drops, the spring expands and applies the brakes. That is why a severe air loss results in a controlled stop rather than a runaway. Normal service braking still comes from air pushing the chamber diaphragm.

Question 13Air Brake System Components and How They Work

What is the caging bolt used for?

  1. Setting the governor cut-in pressure
  2. Holding a spring brake compressed for towing Correct answer.
  3. Locking the slack adjuster in place
  4. Bleeding water from the wet tank

Explanation

When a vehicle has no air pressure, the spring brake stays applied and the wheels will not turn. Inserting and tightening the caging bolt mechanically compresses the spring so the vehicle can be moved or towed. The bolt must be removed before the vehicle returns to normal service.

Question 14Air Brake System Components and How They Work

On a vehicle with spring brakes, what does pulling out the parking control knob do?

  1. It applies the parking brakes Correct answer.
  2. It opens the safety valve
  3. It releases air from the wet tank
  4. It drains the supply tanks

Explanation

The push-pull control valve exhausts air from the spring brake chambers when pulled out, allowing the springs to apply the brakes for parking. Pushing the knob in restores air pressure and releases them. The valve controls only the spring brake circuit, not the tanks or safety valve.

Question 15Air Brake System Components and How They Work

The low air pressure warning must come on before pressure drops below about what level?

  1. 30 psi
  2. 60 psi Correct answer.
  3. 80 psi
  4. 90 psi

Explanation

A warning light and buzzer are required to activate before reservoir pressure falls below about 60 psi, giving the driver time to stop safely before spring brakes apply. The warning is a steady red light plus an audible alert. Below roughly 45 psi the spring brakes may begin to apply on their own.

Question 16Air Brake System Components and How They Work

What does an air dryer do?

  1. It adds alcohol to the brake lines
  2. It cools the compressor during long climbs
  3. It raises the pressure in the wet tank
  4. It removes moisture from the air Correct answer.

Explanation

The air dryer is mounted between the compressor and the reservoirs and removes water vapor before the air reaches the tanks. Keeping air dry reduces corrosion and prevents ice in lines during freezing weather. It does not change pressure or add chemicals to the system.

Question 17Air Brake System Components and How They Work

In an S-cam brake, what does the S-cam do when the pushrod moves?

  1. It pulls the shoes away from the drum
  2. It opens the supply valve in the tank
  3. It spreads the shoes against the drum Correct answer.
  4. It measures the pushrod stroke

Explanation

The pushrod turns the slack adjuster, which rotates the S-cam shaft; the cam lobes push the brake shoes outward against the drum. Release springs pull the shoes back when air pressure is removed. Stroke length is a measurement made by the inspector, not a function of the cam.

Question 18Air Brake System Components and How They Work

What does a double check valve do?

  1. It checks the pressure twice each second
  2. It lets air in from either source, one way Correct answer.
  3. It drains moisture from two lines at once
  4. It doubles the pressure going to the brakes

Explanation

A double check valve has two inlets and one outlet; it allows air to pass from whichever source has the higher pressure while preventing backflow into the other source. It is used where two supply paths must feed a single circuit. It does not multiply pressure or drain anything.

Question 19Air Brake System Components and How They Work

Why is a relay valve used near the rear brakes?

  1. To reduce the pressure the compressor builds
  2. To drain the rear tank automatically
  3. To apply and release the rear brakes faster Correct answer.
  4. To stop air from reaching the front brakes

Explanation

On a long vehicle, air from the foot valve would take too long to travel to the rear chambers. A relay valve uses a small control signal from the foot valve to send air from a nearby reservoir directly to the rear chambers, speeding application and release. It does not regulate compressor output or isolate the front brakes.

Question 20Air Brake System Components and How They Work

What is the alcohol evaporator for?

  1. Making the compressor run cooler
  2. Cleaning the brake drums
  3. Raising the octane of the fuel
  4. Keeping moisture from freezing in lines Correct answer.

Explanation

The alcohol evaporator introduces alcohol vapor into the air system so that any moisture present will not freeze in lines or valves in cold weather. It is a cold-weather aid, and it does not replace draining the tanks. It has nothing to do with fuel or with drum cleaning.

Question 21Air Brake System Components and How They Work

What does the tractor protection valve do?

  1. It balances pressure between front and rear axles
  2. It drains the wet tank each time you stop
  3. It closes the air lines if the trailer breaks away Correct answer.
  4. It slows the compressor when tanks are full

Explanation

The tractor protection valve guards the tractor's air supply. If the trailer separates or its lines rupture, the valve closes the tractor-to-trailer lines so the tractor keeps enough air to stop. Trailer air supply is restored when the control is pushed in with pressure built up.

Question 22Air Brake System Components and How They Work

A pushrod stroke measurement is longer than the adjustment limit. What does this mean?

  1. The brakes are out of adjustment Correct answer.
  2. The compressor is worn out
  3. The air dryer needs a new filter
  4. The governor cut-out is too high

Explanation

Excessive pushrod travel means the chamber must extend too far before the shoes contact the drum, so the brakes are out of adjustment and may not apply fully. The vehicle must be put out of service until the slack adjusters are corrected. Stroke length is a mechanical condition and does not reflect compressor, governor, or dryer problems.

Question 23Air Brake System Components and How They Work

How does a wedge brake apply the shoes?

  1. A wedge is driven between the shoes Correct answer.
  2. Air pressure pushes the shoe directly
  3. A rotating S-cam spreads the shoes
  4. A spring pulls the shoes to the drum

Explanation

In a wedge brake, the pushrod drives a wedge between two shoes or rollers, forcing them apart and into the drum. It achieves the same result as an S-cam without using a rotating cam shaft. The shoes are still released by return springs, but springs do not apply the brake.

Question 24Air Brake System Components and How They Work

Why does a dual air brake system use two separate circuits?

  1. To keep the front brakes cooler on downgrades
  2. To let the trailer brake with the tractor
  3. To give the compressor two places to send air
  4. So one circuit keeps working after a failure Correct answer.

Explanation

Splitting the system into two circuits means a leak or failure in one will not drain the other, so the vehicle still has some braking ability. Check valves isolate the circuits so air cannot escape from the good one into the failed one. The purpose is redundancy, not cooling or trailer control.

Question 25Air Brake System Components and How They Work

How does a foot (treadle) valve control how hard the brakes apply?

  1. It opens fully as soon as the pedal is touched
  2. It lets in more air as the pedal is pressed farther Correct answer.
  3. It sends a fixed pressure once the pedal moves
  4. It sends air only when the pedal is released

Explanation

The foot valve is progressive: the farther the treadle is pushed, the more air pressure it delivers to the brake chambers, and releasing it exhausts that air. This lets the driver modulate braking instead of getting full pressure on every application. It is not an on-off switch.

Question 26Air Brake System Components and How They Work

What is the main job of the air compressor in an air brake system?

  1. To remove moisture from the compressed air
  2. To apply the brakes at each wheel
  3. To build air pressure for the system Correct answer.
  4. To store compressed air for later use

Explanation

The compressor is the source of compressed air; it draws in air and pumps it into the reservoirs until the governor reaches cut-out pressure. Storage is the tank's job, moisture removal is the dryer's job, and brake application is done by the brake chambers.

Question 27Air Brake System Components and How They Work

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

  1. To refill the tank from the compressor
  2. To signal the driver when pressure is low
  3. To drain water from the tank
  4. To release air at excessive pressure Correct answer.

Explanation

The safety valve is a pressure relief device set above normal governor cut-out, commonly around 150 psi. It vents air to protect the tank and system from overpressure if the governor fails.

Question 28Air Brake System Components and How They Work

What does a check valve do in an air brake system?

  1. It limits the maximum pressure in a circuit
  2. It allows air to flow in only one direction Correct answer.
  3. It drains moisture from the reservoir
  4. It speeds up the release of the brakes

Explanation

A check valve is a one-way valve; it permits air to flow in one direction and blocks reverse flow. This protects circuits and reservoirs from backflow.

Question 29Air Brake System Components and How They Work

What is the job of the slack adjuster?

  1. It filters dirt from the air before the chamber
  2. It holds the brake chamber to the axle housing
  3. It releases air from the chamber when the brake is let off
  4. It converts pushrod travel into cam rotation Correct answer.

Explanation

The slack adjuster is a lever on the camshaft that converts the straight push of the brake chamber pushrod into rotation of the S-cam or wedge shaft. It also provides the adjustment point for brake clearance.

Question 30Air Brake System Components and How They Work

How is the stop light switch on a typical air brake vehicle usually operated?

  1. By the rotation of the wheel sensors
  2. By a cable connected to the brake pedal
  3. By air pressure from the brake valve Correct answer.
  4. By the parking brake control knob

Explanation

On air brake vehicles, the stop light switch is commonly a pressure switch operated by air from the service brake valve. When brake pressure rises, the switch closes and turns on the brake lights.

Question 31Air Brake System Components and How They Work

On a tractor-trailer air brake hookup, the emergency (supply) line glad hand is usually what color?

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

Explanation

In the standard color code, the emergency (supply) line is red and the service line is blue. Matching colors helps prevent crossed lines.

Question 32Air Brake System Components and How They Work

What does the supply pressure gauge on the dash show?

  1. The pressure being sent to the trailer brakes
  2. The amount of pushrod travel at each wheel
  3. The pressure stored in the reservoirs Correct answer.
  4. The electrical load on the ABS system

Explanation

The dash supply gauge shows reservoir pressure, letting the driver know whether the system has enough air to operate safely. It does not show application pressure or pushrod travel.

Question 33Air Brake System Components and How They Work

How is the air compressor on most heavy trucks driven?

  1. By a gear or belt from the engine Correct answer.
  2. By an electric motor mounted on the frame
  3. By a separate small gasoline engine
  4. By hydraulic pressure from the steering pump

Explanation

Heavy-truck compressors are usually belt- or gear-driven from the engine, so they run whenever the engine runs. The governor controls whether they are pumping or unloaded.

Question 34Air Brake System Components and How They Work

A Type 30 brake chamber has an effective diaphragm area of about what size?

  1. 15 square inches
  2. 30 square inches Correct answer.
  3. 9 square inches
  4. 24 square inches

Explanation

The type number of a brake chamber is its approximate effective diaphragm area in square inches. A Type 30 chamber therefore has about 30 square inches of effective area and produces more force than a Type 24 or Type 20.

Question 35Air Brake System Components and How They Work

What is the purpose of a quick release valve?

  1. It limits how much pressure reaches the front brakes
  2. It vents air near the chambers to speed release Correct answer.
  3. It drains water from the supply reservoir
  4. It releases the parking brake when air pressure returns

Explanation

A quick release valve is mounted near the brake chambers. When the brake valve releases, it exhausts chamber air locally instead of sending it all the way back through a long line, so the brakes release faster.

Question 36Air Brake System Components and How They Work

When does an automatic slack adjuster normally make its adjustment?

  1. Only when the driver uses the manual adjusting nut
  2. Whenever the emergency line is disconnected
  3. Only when the vehicle is parked and shut off
  4. During normal brake applications Correct answer.

Explanation

An automatic slack adjuster adjusts itself as the brakes are applied during normal driving, maintaining proper pushrod stroke. The manual adjusting nut is for initial setup or repair, not routine adjustment.

Question 37Air Brake System Components and How They Work

What does a front brake limiting valve do on an older vehicle that has one?

  1. It increases front brake force during hard stops
  2. It reduces front brake pressure on slick roads Correct answer.
  3. It keeps the front brakes from releasing at all
  4. It connects the front brakes to the trailer line

Explanation

Some older vehicles have a front brake limiting valve with a 'slippery' setting that reduces front brake pressure to help prevent wheel lockup on poor surfaces. It is not a normal-pressure booster and does not keep brakes applied.

Question 38Air Brake System Components and How They Work

What is the trailer hand valve used for?

  1. To test the trailer ABS system
  2. To apply the trailer brakes by hand Correct answer.
  3. To release the trailer spring brakes
  4. To supply air to the trailer tires

Explanation

The trailer hand valve, sometimes called the trolley valve, lets the driver apply only the trailer brakes by hand. It is useful for checking the trailer brake hookup and for some skid-control techniques.

Question 39Air Brake System Components and How They Work

What is the function of a spring brake priority valve?

  1. It prevents the compressor from overcharging the tanks
  2. It limits the pressure used by the parking brake knob
  3. It releases the trailer brakes before the tractor brakes
  4. It charges the spring brake chambers first Correct answer.

Explanation

A spring brake priority valve directs air to the spring brake chambers before charging other circuits. That ensures the parking brakes can be released before the vehicle is driven.

Question 40Air Brake System Components and How They Work

What does an inversion valve do in a spring brake system?

  1. It lets the driver apply the trailer brakes by hand
  2. It controls spring brake force after a pressure loss Correct answer.
  3. It reverses the flow of air back to the compressor
  4. It raises the pressure above the governor cut-out point

Explanation

An inversion valve provides a controlled, modulated application of the spring brakes when system pressure is lost, rather than an abrupt full application. It also helps prevent compounding with the service brakes.

Question 41Air Brake System Components and How They Work

What does a bobtail proportioning valve do on a tractor?

  1. It balances air pressure between the two circuits
  2. It automatically sets the trailer spring brakes
  3. It reduces rear brake pressure with no trailer Correct answer.
  4. It increases trailer brake pressure when loaded

Explanation

A bobtail proportioning valve reduces the air pressure sent to the tractor's rear brakes when no trailer is connected. This reduces the chance of rear-wheel lockup in a bobtail condition.

Question 42Air Brake System Components and How They Work

What does an ABS controller do when it detects a wheel about to lock during braking?

  1. It applies the parking brake to that wheel
  2. It increases air pressure to that wheel
  3. It modulates pressure to that wheel Correct answer.
  4. It disables the service brakes on that axle

Explanation

ABS uses wheel speed sensors to detect an impending lockup and then modulates brake pressure at that wheel. This helps maintain steering and traction during hard braking.

Question 43Air Brake System Components and How They Work

What is the unloader mechanism on the compressor designed to do?

  1. Increase compressor output at high pressure
  2. Let the compressor idle without pumping air Correct answer.
  3. Drain oil from the compressor crankcase
  4. Cool the compressor discharge air

Explanation

When the governor reaches cut-out pressure, it signals the unloader to hold the compressor intake valves open. The compressor keeps turning but stops pumping air until pressure falls to cut-in.

Question 44Air Brake System Components and How They Work

What returns the brake chamber pushrod when air pressure is released?

  1. The return spring in the brake chamber Correct answer.
  2. The piston inside the air compressor
  3. The spring inside the governor housing
  4. The cable attached to the slack adjuster

Explanation

When air pressure is released from the service chamber, the return spring pulls the pushrod back to the released position. This lets the slack adjuster and cam return, allowing the brake shoes to move away from the drum.

Question 45Air Brake System Components and How They Work

Why is the compressor discharge line often made of coiled tubing?

  1. To measure how much air the compressor moves
  2. To store extra air for brake applications
  3. To cool the air before it reaches the tank Correct answer.
  4. To make the compressor run more quietly

Explanation

The compressor discharge air is hot; a coiled line gives it more surface area and time to cool before it enters the reservoir. The coil also allows some flex between the engine-mounted compressor and the frame-mounted system.

Question 46Air Brake System Components and How They Work

What does the relay emergency valve on a trailer do if the emergency line is broken?

  1. It releases the trailer brakes to allow towing
  2. It seals the break and keeps the brakes released
  3. It applies the trailer brakes automatically Correct answer.
  4. It sends all air pressure to the trailer tires

Explanation

The trailer relay emergency valve senses a loss of emergency line pressure, such as a broken line or disconnected glad hand. It then applies the trailer brakes automatically as a fail-safe.

Question 47Air Brake System Components and How They Work

What is a pressure protection valve designed to do?

  1. Balance pressure between the primary and secondary circuits
  2. Release air from the tank if pressure is too high
  3. Close off nonessential circuits when pressure drops Correct answer.
  4. Raise the governor cut-out pressure

Explanation

A pressure protection valve closes off nonessential accessories when system pressure falls below a set level. This preserves air for the essential brake circuits.

Question 48Air Brake System Components and How They Work

What does 'compounding' mean in a spring brake system?

  1. Service and spring force apply one brake together Correct answer.
  2. The compressor is charging two reservoirs at once
  3. Two trailers are connected to one tractor
  4. The ABS is working on two wheels at the same time

Explanation

Compounding occurs when the service brake and the spring brake both apply force to the same foundation brake at the same time. This can overstress parts, so systems are designed to prevent it.

Question 49Air Brake System Components and How They Work

What does a load-sensing proportioning valve do on a heavy truck?

  1. It varies rear brake pressure with the vehicle load Correct answer.
  2. It increases front brake pressure when the truck is empty
  3. It equalizes pressure between the two air circuits
  4. It sets the parking brakes when the load shifts

Explanation

A load-sensing proportioning valve changes rear brake application pressure according to how heavily the vehicle is loaded. A light load needs less rear brake force to avoid lockup than a heavy load.

Question 50Air Brake System Components and How They Work

If one circuit of a dual air brake system develops a major leak, what keeps the other circuit from draining?

  1. The low pressure warning buzzer sounding
  2. Check valves isolating the failed circuit Correct answer.
  3. The governor shutting off the compressor
  4. The safety valve closing the leaking line

Explanation

Dual air brake systems use check valves and separate reservoirs so a major leak in one circuit does not drain the other. The intact circuit can still provide braking, although the low pressure warning will activate.