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BC Air Brake Knowledge Test study materials
BC Air Brake Knowledge Test study resources

BC Air Brake Knowledge Test practice questions

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Question 1Air Brake System Components and Function

What is the primary purpose of the air compressor in a vehicle air brake system?

  1. To filter moisture and oil from the air supply
  2. To regulate the amount of air sent to each brake
  3. To release air pressure from the brake chambers
  4. To build and maintain air pressure in the reservoirs Correct answer.

Explanation

The compressor is driven by the engine and pumps atmospheric air into the storage reservoirs, maintaining the supply that braking and auxiliary devices depend on. Pressure regulation is the governor's job, and each brake application is controlled by valves downstream of the reservoirs.

Question 2Air Brake System Components and Function

What is the function of the safety valve fitted to an air reservoir?

  1. It closes the supply line if a coupler separates
  2. It vents air if reservoir pressure exceeds a set limit Correct answer.
  3. It drains accumulated moisture automatically
  4. It prevents backflow from the reservoir to the compressor

Explanation

The safety valve is a spring-loaded pop-off valve set above the governor cut-out pressure. If the governor fails and pressure keeps rising, the valve opens and protects the reservoir and other components from over-pressurization.

Question 3Air Brake System Components and Function

The foot (treadle) valve controls service braking by:

  1. directly applying spring force to the slack adjusters
  2. reducing reservoir pressure before it reaches the compressor
  3. routing air only to the trailer service line during a brake application
  4. metering air pressure to the chambers as the pedal is pushed Correct answer.

Explanation

The foot valve meters air from the reservoirs to the service brake chambers in proportion to pedal travel, so the driver controls application pressure. It supplies both tractor and trailer service brakes, and it does not act on the spring brakes or the compressor.

Question 4Air Brake System Components and Function

A spring brake chamber applies the brakes when:

  1. the foot valve is fully released
  2. reservoir pressure is at maximum
  3. air pressure is reduced or lost Correct answer.
  4. the slack adjuster is backed off

Explanation

The spring is held compressed by air pressure whenever the system is charged. When air is deliberately released for parking, or lost through a failure, the spring extends and applies the brakes mechanically.

Question 5Air Brake System Components and Function

In British Columbia, the low air pressure warning device must operate before pressure falls below:

  1. 30 psi (207 kPa)
  2. 90 psi (621 kPa)
  3. 60 psi (414 kPa) Correct answer.
  4. 120 psi (827 kPa)

Explanation

The warning device must alert the driver before reservoir pressure drops below 60 psi. At that point braking performance is seriously reduced, so the driver must stop and find the cause of the pressure loss.

Question 6Air Brake System Components and Function

Why must air reservoirs be drained regularly?

  1. To keep the compressor from cycling too often
  2. To reset the governor to its cut-in pressure
  3. To remove condensed water and oil that can damage valves and brakes Correct answer.
  4. To increase the reservoir's air capacity

Explanation

Compressor air carries moisture and oil that condense in the reservoirs, especially as the air cools. Draining removes this contamination, which would otherwise cause corrosion, freeze-ups, and sticky or damaged valves.

Question 7Air Brake System Components and Function

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

  1. It raises the governor cut-out pressure
  2. It adds alcohol vapour to keep moisture from freezing Correct answer.
  3. It cleans the desiccant in the air dryer
  4. It lubricates the compressor crankshaft

Explanation

An alcohol evaporator introduces alcohol vapour into the air stream so that any moisture condensing in lines and valves is less likely to freeze in cold weather. It is a cold-weather aid and does not replace regular draining or air drying.

Question 8Air Brake System Components and Function

On a tractor-trailer combination, the emergency (supply) glad hand is normally identified by which colour?

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

Explanation

Glad hands follow a standard colour code so the lines are not crossed: red for the emergency (supply) line and blue for the service line. Correct hook-up matters because the supply line charges the trailer and causes its emergency brakes to apply if it separates.

Question 9Air Brake System Components and Function

The governor on an air compressor:

  1. releases air from the brake chambers when the pedal is released
  2. loads and unloads the compressor to hold a set pressure range Correct answer.
  3. balances the air pressure between the tractor and the trailer
  4. limits the maximum pressure delivered to the brake chambers

Explanation

The governor senses reservoir pressure and signals the compressor unloading mechanism. It unloads the compressor at cut-out pressure and lets it build air again at cut-in pressure, keeping the system within its designed range.

Question 10Air Brake System Components and Function

What is the main function of a wet tank in a multi-reservoir air system?

  1. It stores air for the parking brakes only
  2. It is the first reservoir to receive compressor air and collects moisture before air moves on Correct answer.
  3. It equalizes pressure between the front and rear brake circuits
  4. It supplies air directly to the foot valve at a reduced pressure

Explanation

The wet (supply) tank receives hot, moist air directly from the compressor, allowing condensation to form and be trapped there. Air then passes through a check valve to the dry tanks, which supply the brake circuits.

Question 11Air Brake System Components and Function

A check valve installed between the wet tank and a dry tank allows air to:

  1. escape to atmosphere when pressure is too high
  2. flow only from the wet tank into the dry tank Correct answer.
  3. flow only from the dry tank back to the wet tank
  4. flow in both directions equally

Explanation

A check valve is a one-way valve. In this location it lets charged air move from the wet tank to the dry tank while preventing air from escaping the dry tank back through the supply side if pressure there is lost.

Question 12Air Brake System Components and Function

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

  1. It reduces application pressure to the front brakes
  2. It converts hydraulic pressure into air pressure
  3. It speeds up applying and releasing air at the brake chambers Correct answer.
  4. It holds the trailer brakes applied when the supply line is broken

Explanation

A relay valve is mounted close to the chambers it serves and uses a small control signal from the foot valve to open a large supply passage. This shortens application and release times by avoiding long lines carrying high-volume air flow.

Question 13Air Brake System Components and Function

A quick release valve is installed near a brake chamber primarily to:

  1. automatically adjust the pushrod stroke
  2. speed up the release of air from the chamber Correct answer.
  3. increase the maximum application pressure at the chamber
  4. prevent the chamber from being applied if a line breaks

Explanation

A quick release valve senses the drop in control pressure when the brake is released and exhausts chamber air locally instead of sending it all the way back through the foot valve. This shortens release time and reduces brake drag.

Question 14Air Brake System Components and Function

What happens inside a desiccant-type air dryer when the compressor unloads?

  1. A purge cycle expels collected moisture to the atmosphere Correct answer.
  2. The desiccant is replaced automatically
  3. Air is diverted directly to the brake chambers
  4. The governor cut-out pressure is raised temporarily

Explanation

When the governor unloads the compressor, a purge valve opens and a burst of dry air carries collected water and oil out of the dryer. This regenerates the desiccant so it can absorb moisture during the next charging cycle.

Question 15Air Brake System Components and Function

The tractor protection valve is designed to:

  1. close the trailer supply line if trailer pressure drops or lines separate Correct answer.
  2. limit tractor speed when trailer pressure is low
  3. automatically apply the trailer spring brakes during normal braking
  4. prevent air from entering the trailer supply line during charging

Explanation

The tractor protection valve closes when supply pressure falls, for example if a coupler separates or a trailer line breaks. It keeps the tractor's remaining air available for its own brakes while the loss of supply causes the trailer emergency brakes to apply.

Question 16Air Brake System Components and Function

The trailer hand valve (trolley valve) in the cab is used to:

  1. release the trailer spring brakes for towing
  2. charge the trailer reservoirs from the tractor
  3. supply air to the trailer service brakes only during a parking check
  4. apply the trailer service brakes independently of the tractor brakes Correct answer.

Explanation

The trailer hand valve is a separate cab control that applies only the trailer service brakes. Drivers use it to check that trailer brakes apply and release, but it must never be relied on as a parking brake because it can bleed off.

Question 17Air Brake System Components and Function

A slack adjuster that is 'out of adjustment' means that:

  1. the reservoir pressure is below cut-in
  2. the pushrod stroke is longer than the manufacturer's limit Correct answer.
  3. the brake chamber has lost its return spring
  4. the brake shoe lining is too thick

Explanation

Pushrod stroke grows as linings and drums wear. When the stroke at full application exceeds the limit specified for that chamber type, the slack adjuster is out of adjustment and braking force and timing are affected.

Question 18Air Brake System Components and Function

When checking pushrod stroke, the measurement is taken with the brakes:

  1. applied only by the spring brake
  2. released and the reservoirs drained
  3. partially applied at about 20 psi
  4. fully applied at normal operating pressure Correct answer.

Explanation

Stroke is measured with the system at normal operating pressure and the brake fully applied, comparing the released and applied pushrod positions. The measured stroke is then compared with the limit for that chamber size and type.

Question 19Air Brake System Components and Function

An automatic slack adjuster maintains correct brake adjustment by:

  1. adjusting itself during normal brake applications and releases Correct answer.
  2. bleeding air from the chamber when the stroke is too long
  3. raising the governor cut-out pressure as linings wear
  4. locking the pushrod when the chamber is at full stroke

Explanation

An automatic slack adjuster takes up clearance during the release stroke whenever the stroke exceeds its designed setting. It works correctly only if it is properly installed with the correct pushrod length and is free of damage.

Question 20Air Brake System Components and Function

A double check valve (two-way check valve) is used to:

  1. let either of two air sources supply a circuit without backflow Correct answer.
  2. allow air to flow in only one direction from the compressor
  3. split reservoir pressure equally between two circuits
  4. release excess pressure to the atmosphere

Explanation

A double check valve has two inlets and one outlet; it passes whichever inlet has the higher pressure and seals the other. It is used where either of two control signals must be able to operate the same circuit without feeding back into the other.

Question 21Air Brake System Components and Function

How does a typical governor unload a compressor when cut-out pressure is reached?

  1. It opens a bleed valve that vents reservoir pressure continuously
  2. It holds the compressor inlet valves open so no air is pumped Correct answer.
  3. It disengages the compressor drive belt using an automatic clutch
  4. It closes the compressor discharge line with an electric solenoid

Explanation

At cut-out the governor sends air to the compressor unloading mechanism, which holds the inlet valves open so the compressor stops pumping even though it keeps turning. A check valve keeps reservoir air from flowing back through the unloaded compressor.

Question 22Air Brake System Components and Function

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

  1. It reverses the direction of air flow through the air dryer
  2. It converts spring brake application into a service brake release
  3. It provides modulated spring brake apply if service air is lost Correct answer.
  4. It inverts the polarity of the low pressure warning switch

Explanation

The inversion valve senses loss of service brake pressure and allows the spring brakes to apply in a modulated way, so the vehicle can still be controlled rather than receiving a full spring brake application. It is part of the emergency braking function on vehicles equipped with spring brakes.

Question 23Air Brake System Components and Function

A pressure protection valve is set to open at about 70 psi. Its purpose is to:

  1. prevent the compressor from exceeding 70 psi
  2. release air from the wet tank at 70 psi
  3. stop supplying non-essential circuits so brake pressure is preserved Correct answer.
  4. apply the spring brakes at 70 psi

Explanation

The pressure protection valve closes off auxiliary circuits such as air seats or suspensions when system pressure falls below its setting. This preserves the remaining air for the brake system so braking is not lost to non-essential loads.

Question 24Air Brake System Components and Function

Why is an anti-compounding valve used on a vehicle with spring brakes?

  1. To allow spring and service brakes to apply together for maximum force
  2. To stop the compressor from charging the spring brake circuit
  3. To prevent service air from adding to spring force when both apply Correct answer.
  4. To prevent the spring from being released when air pressure is high

Explanation

If service brake air is applied while the spring is already applying the brake, the two forces add together and can bend the pushrod or damage the chamber. The anti-compounding valve prevents service air from reaching the chamber while the spring brake is applied.

Question 25Air Brake System Components and Function

A spring brake must be caged when:

  1. the vehicle is parked on a level surface for a short time
  2. the reservoir pressure exceeds governor cut-out
  3. the automatic slack adjuster is being replaced
  4. the spring brake chamber must be released without air pressure, such as for towing or repair Correct answer.

Explanation

Caging uses a mechanical bolt to compress the spring so the brake releases without air pressure, which is needed for towing a disabled vehicle or servicing the chamber. The caging bolt must be backed off before the vehicle returns to service or the spring brake will not function.

Question 26Air Brake System Components and Function

In an air brake system, the compressor is used to:

  1. Remove moisture from the compressed air
  2. Compress air and supply it to the reservoirs Correct answer.
  3. Store a reserve of compressed air for the brakes
  4. Control the pressure applied to each brake chamber

Explanation

The compressor takes in atmospheric air, compresses it, and delivers it to the reservoirs through the air dryer and lines. Storing air, controlling application pressure at the wheels, and removing moisture are handled by other components.

Question 27Air Brake System Components and Function

A safety valve fitted to an air reservoir is intended to:

  1. Release air if reservoir pressure rises above a set limit Correct answer.
  2. Prevent air from flowing back toward the compressor
  3. Drain water from the bottom of the reservoir
  4. Signal the driver when system pressure is low

Explanation

A safety valve is a pressure-relief device set above normal governor cut-out; it opens if pressure control fails and reservoir pressure climbs too high. It is not a drain, warning device, or one-way flow valve.

Question 28Air Brake System Components and Function

The yellow, diamond-shaped push-pull valve on the dash controls:

  1. The trailer service brakes only
  2. The compressor cut-in pressure
  3. The front brake limiting function
  4. The parking and emergency spring brakes Correct answer.

Explanation

The yellow diamond push-pull valve is the parking brake control; pulling it out exhausts air to apply the spring brakes and pushing it in charges the spring brake circuit to release them. It does not control trailer service brakes, governor settings, or front limiting.

Question 29Air Brake System Components and Function

When the brake pedal is pressed, the foot valve (treadle valve):

  1. Meters air pressure to the brake chambers according to pedal position Correct answer.
  2. Closes the supply line so the trailer brakes can release
  3. Sends a constant-pressure signal to the relay valve regardless of pedal travel
  4. Releases air from the brake chambers to apply the brakes

Explanation

The foot valve is a modulating valve: pedal travel and force determine how much air pressure is delivered to the brake chambers. It does not release air to apply brakes, send a fixed signal, or close the supply line.

Question 30Air Brake System Components and Function

A dual-circuit foot valve is used so that:

  1. The parking brakes release automatically when the pedal is pressed
  2. Each service brake circuit can still operate if the other circuit fails Correct answer.
  3. The trailer brakes always apply before the tractor brakes
  4. Compressor output is divided equally between the wet and dry tanks

Explanation

A dual-circuit foot valve has separate supply and delivery sections so a failure in one service circuit does not disable the other. This gives redundant, partial braking capability rather than automatic parking brake release or fixed proportioning.

Question 31Air Brake System Components and Function

A brake chamber marked "Type 30" means that the chamber has:

  1. A 30-to-1 slack adjuster ratio
  2. An effective diaphragm area of 30 square inches Correct answer.
  3. A 30-inch pushrod stroke
  4. A maximum working pressure of 30 psi

Explanation

The type number of a brake chamber is its effective diaphragm area in square inches. A type 30 chamber therefore produces more force for a given air pressure than a type 24 or type 20.

Question 32Air Brake System Components and Function

The bobtail proportioning valve on a tractor is designed to:

  1. Increase trailer brake pressure when the trailer is loaded
  2. Reduce drive-axle braking force when running without a trailer Correct answer.
  3. Prevent the tractor from rolling when the parking brake is released
  4. Release the trailer spring brakes during a bobtail run

Explanation

Without a trailer, the tractor's drive axles carry much less weight, so full drive-axle brake pressure can lock the wheels. The bobtail proportioning valve reduces that pressure when no trailer is connected.

Question 33Air Brake System Components and Function

A front brake limiting valve, when fitted, is used to:

  1. Balance air pressure between the front and rear service circuits
  2. Hold the front brakes off while the parking brake is applied
  3. Apply the front brakes automatically on a slippery surface
  4. Reduce front brake pressure to help prevent wheel lock-up Correct answer.

Explanation

A front brake limiting valve can reduce application pressure to the steer axle to lessen the chance of front wheel lock-up, especially on slippery surfaces. It does not apply brakes automatically or balance circuit pressure.

Question 34Air Brake System Components and Function

The emergency relay valve on a trailer:

  1. Controls the trailer spring brakes and applies them if supply is lost Correct answer.
  2. Sets the compressor cut-out pressure for the trailer system
  3. Allows the driver to release the trailer brakes from the cab
  4. Filters moisture from the air before it reaches the trailer reservoir

Explanation

On many trailers the emergency relay valve controls the spring brake chambers and responds to loss of supply pressure by applying the spring brakes. It does not dry air, set governor pressure, or serve as the cab control.

Question 35Air Brake System Components and Function

An application pressure gauge shows:

  1. The pressure being delivered to the brakes Correct answer.
  2. The pressure remaining in the trailer supply line
  3. The pressure in the wet tank only
  4. The compressor's cut-in pressure

Explanation

An application pressure gauge displays the pressure being delivered to the brake chambers during a brake application, which helps the driver judge how hard the brakes are being applied. It is not a reservoir, governor, or supply-line gauge.

Question 36Air Brake System Components and Function

A manual drain valve on an air reservoir is operated to:

  1. Add air to the reservoir from an outside source
  2. Test the safety valve's opening pressure
  3. Adjust the governor cut-out setting
  4. Release air and moisture from the reservoir Correct answer.

Explanation

The manual drain valve at the bottom of a reservoir is opened to blow out air and accumulated water and oil. It is not a filling point, an adjustment for the governor, or a safety-valve test fitting.

Question 37Air Brake System Components and Function

The purpose of a trailer spring brake priority valve is to:

  1. Supply the trailer service brakes before the tractor service brakes
  2. Apply the trailer spring brakes before the tractor parking brake
  3. Allow the trailer spring brakes to be released by the service line
  4. Delay release of the trailer spring brakes until supply pressure is sufficient Correct answer.

Explanation

The trailer spring brake priority valve delays release of the trailer spring brakes until the trailer supply pressure is high enough to hold them off safely. This prevents dragging brakes and ensures the trailer has adequate air before it rolls.

Question 38Air Brake System Components and Function

A truck with a dual-circuit air system has a total loss of pressure in the front service circuit. What should the driver expect?

  1. All service brakes will be lost and the spring brakes will apply immediately
  2. The front brakes will apply automatically and the rear brakes will release
  3. The compressor will stop charging the rear circuit
  4. The rear service brakes will still operate, but stopping distance will increase Correct answer.

Explanation

In a dual-circuit system, a total loss in one service circuit leaves the other circuit available, though braking is reduced and stopping distance increases. The spring brakes do not apply just because one service circuit failed, and the compressor still charges the remaining circuit.

Question 39Air Brake System Components and Function

Reservoir pressure stays below cut-out and the compressor runs continuously. The governor is functioning correctly and there are no external leaks. Which condition could explain this?

  1. The application pressure gauge is reading high
  2. The safety valve is stuck closed
  3. The low pressure warning switch is faulty
  4. The compressor unloader valves are stuck open Correct answer.

Explanation

If the unloader valves remain open, the compressor keeps pumping air back through the intake instead of building pressure, even though the governor is calling for load. This mechanical fault prevents the compressor from delivering air to the reservoirs.

Question 40Air Brake System Components and Function

A check valve is often fitted in the compressor discharge line. Its purpose is to:

  1. Trap moisture so it can be drained at the compressor
  2. Allow the governor to sense wet-tank pressure
  3. Limit the compressor's maximum output pressure
  4. Prevent reservoir air from flowing back to the compressor Correct answer.

Explanation

When the governor unloads the compressor, the discharge line check valve closes so reservoir air cannot bleed back through the compressor. This protects system pressure and prevents unnecessary compressor cycling. The valve does not trap moisture or limit maximum pressure.

Question 41Air Supply, Compressors and Reservoirs

Which component controls when the air compressor loads and unloads to maintain system pressure?

  1. The safety valve
  2. The governor Correct answer.
  3. The air dryer
  4. The check valve

Explanation

The governor senses reservoir pressure and signals the compressor unloader. It normally unloads the compressor at about 105 psi and lets it load again at about 85 psi.

Question 42Air Supply, Compressors and Reservoirs

The safety valve on an air reservoir is normally set to open at approximately what pressure?

  1. 125 psi
  2. 105 psi
  3. 150 psi Correct answer.
  4. 85 psi

Explanation

The safety valve is a backup relief device that protects the system if the governor fails to unload the compressor. It is typically set to open at about 150 psi.

Question 43Air Supply, Compressors and Reservoirs

When should moisture be drained from air reservoirs?

  1. Only when the low-pressure warning sounds
  2. Only after the air dryer is replaced
  3. Once every month
  4. At the end of each day Correct answer.

Explanation

Moisture and oil collect in the reservoirs and must be drained daily, preferably at the end of the day. Regular draining helps prevent freeze-up and corrosion in the air system.

Question 44Air Supply, Compressors and Reservoirs

Which reservoir receives air directly from the compressor and collects most of the moisture and oil?

  1. A spring brake chamber
  2. The air filter housing
  3. The dry reservoir
  4. The wet reservoir Correct answer.

Explanation

The first reservoir after the compressor is commonly called the supply or wet reservoir. Air cools there, so most moisture and oil drop out before the air moves to the dry reservoir.

Question 45Air Supply, Compressors and Reservoirs

What is the main job of the air dryer in the air supply system?

  1. To cool the brake drums
  2. To raise system pressure
  3. To remove moisture from the air Correct answer.
  4. To lubricate the compressor

Explanation

The air dryer removes water vapour from the compressed air before it reaches the reservoirs. This helps prevent corrosion and winter freeze-up in the air brake system.

Question 46Air Supply, Compressors and Reservoirs

The low-pressure warning device must warn the driver before system pressure drops below what value?

  1. 40 psi
  2. 60 psi Correct answer.
  3. 90 psi
  4. 20 psi

Explanation

The low-pressure warning must activate before pressure falls below about 60 psi. This gives the driver time to stop safely before spring brakes apply at a much lower pressure.

Question 47Air Supply, Compressors and Reservoirs

In most heavy vehicles, the air compressor is driven by which power source?

  1. The engine Correct answer.
  2. Exhaust gas pressure
  3. Wheel rotation
  4. An electric motor

Explanation

The air compressor is normally mounted on the engine and driven by gears or a belt from the engine. It therefore operates whenever the engine is running.

Question 48Air Supply, Compressors and Reservoirs

What is the purpose of a check valve between the supply reservoir and the dry reservoir?

  1. To allow air to flow both ways
  2. To limit pressure to 60 psi
  3. To drain water automatically
  4. To stop backflow to the supply tank Correct answer.

Explanation

A check valve allows air to flow in one direction only. It prevents air from flowing back toward the supply reservoir when pressure in the downstream tank is higher.

Question 49Air Supply, Compressors and Reservoirs

The system pressure climbs well above the normal cut-out pressure and the safety valve releases. What is the most likely cause?

  1. A leaking check valve
  2. A plugged drain cock
  3. A restricted air intake
  4. A faulty governor Correct answer.

Explanation

The governor should unload the compressor at the cut-out pressure. If it fails to do so, pressure continues to rise until the safety valve opens to protect the system.

Question 50Air Supply, Compressors and Reservoirs

The compressor builds pressure very slowly and a visual check shows a loose compressor drive belt. What should be done?

  1. Replace the safety valve
  2. Increase the governor cut-out pressure
  3. Tighten or replace the belt Correct answer.
  4. Open the drain cocks and leave them open

Explanation

A loose or slipping drive belt reduces compressor speed and output. Correcting belt tension or replacing a worn belt is the appropriate repair before further testing.