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Ohio CDL Air Brakes Test study materials
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Ohio CDL Air Brakes Test practice questions

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Question 1Air Brake System Components and Their Functions

On a standard air brake system, what is the primary job of the compressor?

  1. It releases air from the spring brakes during parking
  2. It measures the amount of air flowing to the chambers
  3. It builds and maintains air pressure in the storage tanks Correct answer.
  4. It converts air pressure into mechanical force at the wheel

Explanation

The compressor is the source of air pressure for the whole system. It is driven by the engine and pumps air into the supply reservoir until the governor cuts it out at the set pressure. Everything else in the system depends on this stored pressure.

Question 2Air Brake System Components and Their Functions

What component controls when the compressor stops and resumes building air pressure?

  1. The alcohol evaporator
  2. The governor Correct answer.
  3. The low pressure warning switch
  4. The safety valve

Explanation

The governor senses reservoir pressure and signals the compressor to cut out at the maximum setting and cut back in at the minimum. This keeps pressure within the operating range. The safety valve is a backup that only releases air if pressure climbs too high.

Question 3Air Brake System Components and Their Functions

What is the function of the safety valve installed on the air reservoir?

  1. It releases air if system pressure exceeds a safe limit Correct answer.
  2. It drains moisture from the tank each day
  3. It warns the driver when pressure is low
  4. It prevents air from flowing back toward the compressor

Explanation

The safety valve is a spring-loaded relief device that opens if pressure rises above its setting, protecting the tanks and lines from over-pressurization. It normally stays closed. If it releases air in service, the system needs inspection.

Question 4Air Brake System Components and Their Functions

Which component applies the brakes when the driver presses the brake pedal?

  1. The quick release valve
  2. The slack adjuster
  3. The supply reservoir
  4. The foot valve (treadle valve) Correct answer.

Explanation

The foot valve, also called the treadle valve, meters air from the reservoirs to the brake chambers in proportion to pedal travel. It is the driver's primary control over braking force. The slack adjuster only transmits that force to the pushrod and cam.

Question 5Air Brake System Components and Their Functions

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

  1. It houses the low pressure warning buzzer
  2. It holds hydraulic fluid for the service brakes
  3. It filters oil out of the compressor discharge
  4. It stores compressed air for use by the brake system Correct answer.

Explanation

The supply reservoir, often called the wet tank, receives air directly from the compressor and stores it. Air then flows to the other reservoirs that feed the brake circuits. Storage capacity allows several brake applications even if the compressor stops.

Question 6Air Brake System Components and Their Functions

What does the low pressure warning device in the cab signal to the driver?

  1. That the compressor has reached its cut-out pressure
  2. That the parking brake is fully released
  3. That the air dryer is purging moisture
  4. That system air pressure has dropped below a safe operating level Correct answer.

Explanation

The low pressure warning switch activates a buzzer and light when reservoir pressure falls below the required threshold, alerting the driver to stop safely. It is a warning, not a shutdown or a normal cycling event. Drivers should treat it as an emergency.

Question 7Air Brake System Components and Their Functions

What is the function of the brake chamber?

  1. It stores air for repeated brake applications
  2. It converts air pressure into mechanical force that applies the brakes Correct answer.
  3. It regulates compressor cut-in and cut-out pressure
  4. It drains contaminants from the air system

Explanation

The brake chamber contains a diaphragm and pushrod. When air pressure enters, it pushes the diaphragm and extends the pushrod, which moves the slack adjuster and applies the brake. It is the point where air pressure becomes mechanical force.

Question 8Air Brake System Components and Their Functions

What does the slack adjuster do in an air brake system?

  1. It releases air from the chamber when the pedal is released
  2. It links the pushrod to the camshaft and adjusts for lining wear Correct answer.
  3. It warns the driver of low air pressure
  4. It controls the pressure delivered to each brake chamber

Explanation

The slack adjuster is a lever on the camshaft spline that the pushrod pushes. It multiplies pushrod force and takes up clearance as linings wear. Pushrod stroke length is the practical measure of its adjustment.

Question 9Air Brake System Components and Their Functions

A driver hears a burst of air from the air dryer shortly after the compressor reaches cut-out pressure. What does this indicate?

  1. The compressor has failed and must be replaced
  2. The safety valve is relieving over-pressure
  3. The dryer is purging collected moisture, which is normal Correct answer.
  4. A brake chamber diaphragm has ruptured

Explanation

Air dryers purge to the atmosphere at the end of each compressor cycle, carrying collected moisture with them. This short burst at cut-out is expected operation. A continuous leak or a burst at other times would be cause for inspection.

Question 10Air Brake System Components and Their Functions

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

  1. To introduce alcohol vapor that helps prevent moisture from freezing in the system Correct answer.
  2. To lubricate the brake chamber diaphragms
  3. To increase compressor output at high altitude
  4. To reduce the noise of the compressor

Explanation

Alcohol evaporators feed alcohol vapor into the air system so that condensed moisture is less likely to freeze in valves and lines in cold weather. They supplement, not replace, draining and air drying. Correct fluid and level matter for safety.

Question 11Air Brake System Components and Their Functions

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

  1. It prevents the trailer from rolling away when parked
  2. It speeds up application and release of the trailer brakes by using a nearby reservoir Correct answer.
  3. It connects the tractor and trailer electrical circuits
  4. It reduces reservoir pressure to the trailer

Explanation

A relay valve uses a control signal from the foot valve to admit air from a nearby reservoir directly to the trailer brake chambers. This shortens the distance air must travel, improving response time. It does not change the pressure commanded by the driver.

Question 12Air Brake System Components and Their Functions

What is the function of the quick release valve?

  1. It opens the safety valve during over-pressure
  2. It exhausts chamber air quickly to speed brake release Correct answer.
  3. It bypasses the air dryer during cold weather
  4. It dumps trailer air if the supply line breaks

Explanation

The quick release valve is mounted near the brake chambers and vents their air locally when the control pressure drops. This shortens the path air must travel to escape, so brakes release faster. Faster release reduces drag and heat buildup.

Question 13Air Brake System Components and Their Functions

What does the tractor protection valve do in a combination vehicle air brake system?

  1. It prevents the tractor from starting unless the trailer is connected
  2. It closes off the tractor air supply and applies the trailer brakes if the trailer breaks away or a line separates Correct answer.
  3. It balances air pressure between the steer and drive axles
  4. It limits tractor air pressure to the trailer to a fixed low value

Explanation

The tractor protection valve senses loss of trailer supply pressure and closes to keep the tractor's air from escaping. It also signals the trailer to apply its spring brakes. This protects the tractor's ability to stop if the trailer separates.

Question 14Air Brake System Components and Their Functions

What is the purpose of the front brake limiting valve found on some older vehicles?

  1. It increases front braking force on slippery roads
  2. It automatically applies the front brakes when the parking brake is set
  3. It reduces front brake application pressure under certain conditions Correct answer.
  4. It balances air pressure between left and right front chambers

Explanation

A front brake limiting valve was intended to reduce front brake force to lessen the risk of wheel lockup, often with a 'slippery' setting. It does not increase braking. Modern systems rely on antilock braking rather than limiting valves.

Question 15Air Brake System Components and Their Functions

What does the spring brake (parking brake) chamber do when air pressure is released from it?

  1. It vents air to the atmosphere through the safety valve
  2. It releases the service brake
  3. It applies the brake by spring force Correct answer.
  4. It increases the slack adjuster free play

Explanation

Spring brakes are held applied by heavy springs and released by air pressure. When air is released, the springs push the pushrod out and apply the brake. This is why a loss of air pressure results in automatic braking.

Question 16Air Brake System Components and Their Functions

A driver notices that the air pressure gauge shows pressure building slowly and the compressor runs almost constantly. Which component is the most likely cause?

  1. A failed low pressure warning switch
  2. A misadjusted slack adjuster
  3. A leaking brake chamber diaphragm or line Correct answer.
  4. A clogged air dryer purge orifice

Explanation

A leak in a chamber, hose, or fitting lets air escape as fast as the compressor supplies it, so pressure builds slowly and the compressor cycles often. The gauge behavior points to a system leak. Slack adjustment affects stroke, not pressure rise.

Question 17Air Brake System Components and Their Functions

What is the function of the one-way check valve in an air brake system?

  1. It regulates the pressure in each reservoir
  2. It warns the driver of low pressure
  3. It allows air to flow in only one direction Correct answer.
  4. It releases air when pressure is too high

Explanation

Check valves permit air to flow one way and block reverse flow. They protect against air loss from one circuit or reservoir back through another. They are common between the compressor and the supply reservoir and between reservoirs.

Question 18Air Brake System Components and Their Functions

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

  1. It increases pressure to the trailer circuit
  2. It drains the wet tank automatically
  3. It closes off a circuit that has lost pressure so other circuits can still build pressure Correct answer.
  4. It applies the parking brakes when the engine is off

Explanation

A pressure protection valve isolates a failed or leaking circuit once pressure drops below a set value. This preserves air in the remaining circuits so the vehicle can still be stopped. It is a safety feature of split or multi-circuit systems.

Question 19Air Brake System Components and Their Functions

A driver performs a static leak test and finds that pressure drops faster than the allowable rate. What does this most directly indicate?

  1. The compressor is over-speeding
  2. The governor cut-out pressure is set too high
  3. There is a leak somewhere in the air system Correct answer.
  4. The safety valve is stuck closed

Explanation

Excessive pressure loss with the brakes applied and the engine off indicates air escaping somewhere in the system. The test isolates the system from the compressor so leaks can be detected. Finding and repairing the leak restores safe operation.

Question 20Air Brake System Components and Their Functions

What is the function of the brake pedal (treadle) free play in a foot valve?

  1. It adjusts the slack adjuster automatically
  2. It allows the valve to fully exhaust air before the brakes apply Correct answer.
  3. It locks the parking brake when the pedal is released
  4. It increases the maximum pressure delivered to the chambers

Explanation

Free play ensures the foot valve returns fully to the exhaust position so chamber air can escape and the brakes release. Without it, brakes may drag and overheat. Free play is set by the valve and linkage design, not by the driver.

Question 21Air Brake System Components and Their Functions

A vehicle with automatic slack adjusters has one chamber with a pushrod stroke longer than the others. What is the most appropriate interpretation?

  1. Automatic slack adjusters require no inspection, so the difference can be ignored
  2. The alcohol evaporator is overfilling the system
  3. The long stroke may indicate a worn or damaged automatic slack adjuster or foundation brake problem that needs inspection Correct answer.
  4. The governor is set too low and should be raised

Explanation

Automatic slack adjusters still need inspection; a stroke longer than the others can signal a slipping or damaged adjuster, worn linings, or a mechanical problem. Stroke differences between chambers on the same axle are a warning sign. The vehicle should be inspected before continued service.

Question 22Air Brake System Components and Their Functions

Why do some air brake systems include a dual-circuit (split) reservoir arrangement?

  1. To supply air to the tires for inflation
  2. To reduce the number of brake chambers needed
  3. To provide separate air supplies so a failure in one circuit does not disable all braking Correct answer.
  4. To allow the compressor to run at two speeds

Explanation

Split systems divide the air supply into separate circuits, commonly front and rear. If one circuit loses pressure, the other can still apply some brakes. This redundancy is a key safety feature of modern air brake systems.

Question 23Air Brake System Components and Their Functions

During a walk-around inspection, a driver hears air escaping at a brake chamber only when the brakes are applied, and it stops when released. What does this suggest?

  1. A stuck-open quick release valve
  2. A failed low pressure warning switch
  3. A misadjusted governor cut-in pressure
  4. A leaking chamber diaphragm or a loose hose fitting on the pressure side Correct answer.

Explanation

Air escaping only during application points to a leak in the pressurized side, such as a diaphragm, hose, or fitting. Release stops the flow because pressure is removed. This condition reduces braking force and must be repaired before driving.

Question 24Air Brake System Components and Their Functions

If the compressor's governor fails so that the compressor never cuts out, which component is designed to protect the system from over-pressurization?

  1. The low pressure warning switch
  2. The relay valve
  3. The quick release valve
  4. The safety valve on the reservoir Correct answer.

Explanation

The safety valve is the last line of defense against over-pressure when the governor fails to unload the compressor. It opens at its set pressure and vents air. A system that regularly vents through the safety valve needs immediate service.

Question 25Air Brake System Components and Their Functions

What does the application pressure gauge on some air brake vehicles show the driver?

  1. The pressure inside the supply reservoir only
  2. The compressor cut-in pressure setting
  3. The amount of moisture in the air tanks
  4. The pressure being delivered to the brake chambers during a brake application Correct answer.

Explanation

An application pressure gauge displays the air pressure being sent to the brake chambers as the driver applies the brakes. It helps the driver judge how hard the brakes are being applied. Reservoir and system pressures are shown on separate gauges.

Question 26Air Brake System Components and Their Functions

On a typical air brake system, what component draws in outside air and compresses it to build system pressure?

  1. The treadle valve
  2. The governor
  3. The compressor Correct answer.
  4. The reservoir

Explanation

The compressor is the engine-driven pump that takes in atmospheric air and compresses it into the system. The governor only controls when the compressor loads and unloads, while reservoirs merely store the compressed air.

Question 27Air Brake System Components and Their Functions

What is the main job of the supply reservoir (wet tank) in a tractor-trailer air brake system?

  1. To receive air directly from the compressor and trap moisture and oil before it reaches the other tanks Correct answer.
  2. To store air that has already passed through the alcohol evaporator
  3. To hold the spring brakes released during normal operation
  4. To meter brake application pressure to the rear axle only

Explanation

The supply, or wet, tank is the first tank the compressor discharges into, so it collects condensate and oil carryover before air moves to the dry tanks. It also serves as a volume that helps smooth pressure fluctuations.

Question 28Air Brake System Components and Their Functions

Which component senses reservoir pressure and tells the compressor when to stop building pressure?

  1. The governor Correct answer.
  2. The safety valve
  3. The low pressure warning switch
  4. The inversion valve

Explanation

The governor monitors system pressure and unloads the compressor at cut-out pressure, then reloads it at cut-out pressure minus the cut-in differential. This cycling keeps pressure within the designed operating range.

Question 29Air Brake System Components and Their Functions

What does the safety valve on an air brake reservoir do?

  1. It limits how fast the brakes can be released
  2. It drains moisture from the tank each time the compressor cuts in
  3. It releases air automatically if system pressure rises above a preset maximum Correct answer.
  4. It holds air in the tank when the engine is shut down

Explanation

The safety valve is a spring-loaded relief device set above normal cut-out pressure. If the governor fails and pressure keeps climbing, the valve vents air to prevent tank or line damage.

Question 30Air Brake System Components and Their Functions

A driver applies the brakes and the pushrod of one brake chamber extends, but the slack adjuster does not rotate to apply the brake. Which component is most likely at fault?

  1. The connection between the pushrod and the slack adjuster Correct answer.
  2. The governor
  3. The brake chamber diaphragm
  4. The relay valve

Explanation

The pushrod transfers force to the slack adjuster, which converts linear motion into rotation of the camshaft. If the rod moves but the adjuster does not, the linkage between them has failed or disconnected.

Question 31Air Brake System Components and Their Functions

What is the purpose of the brake chamber return spring?

  1. To keep the diaphragm sealed against the chamber housing
  2. To pull the pushrod back and release the brake when air pressure is removed Correct answer.
  3. To hold the slack adjuster against the camshaft
  4. To maintain pressure in the reservoir during a brake application

Explanation

When the foot valve is released, air exhausts from the chamber and the return spring retracts the pushrod. This allows the slack adjuster and camshaft to return so the brake shoes clear the drum.

Question 32Air Brake System Components and Their Functions

In a system with spring brakes, what is the function of the inversion valve?

  1. It reverses the direction of pushrod travel inside the chamber
  2. It converts supply air into a vacuum signal for the relay valve
  3. It provides a modulated spring brake application when pressure in one circuit is lost Correct answer.
  4. It prevents air from flowing back into the compressor discharge line

Explanation

The inversion valve allows a graduated spring brake application when a service circuit fails, so the vehicle can still be stopped with some modulation rather than a full emergency application. It senses loss of pressure and routes air accordingly.

Question 33Air Brake System Components and Their Functions

What does the brake light switch in an air brake system sense to turn on the stop lamps?

  1. Rotation of the slack adjuster
  2. Current draw of the compressor clutch
  3. Movement of the brake pedal or rise in application air pressure Correct answer.
  4. Pressure in the supply reservoir only

Explanation

The stop lamp switch is triggered either mechanically by pedal movement or by a pressure signal from the service brake circuit. Either way, it closes the circuit to the brake lights when the brakes are applied.

Question 34Air Brake System Components and Their Functions

Why does an air brake system include a drain valve at the bottom of each reservoir?

  1. To supply air to the brake light switch
  2. To release excess pressure after the compressor cuts out
  3. To allow accumulated water and oil to be blown out of the tank Correct answer.
  4. To equalize pressure between the primary and secondary circuits

Explanation

Moisture condenses inside reservoirs as compressed air cools, and oil carryover from the compressor can collect there too. Draining each tank periodically removes these contaminants before they reach valves and brake components.

Question 35Air Brake System Components and Their Functions

A driver notices that the air pressure gauge climbs to about 130 psi and the safety valve vents air. What does this most likely indicate?

  1. The governor has failed to unload the compressor Correct answer.
  2. The compressor is worn and cannot build pressure
  3. The drain valves are all open
  4. The low pressure warning switch is stuck closed

Explanation

Normal cut-out pressure is well below the safety valve setting, so venting at high pressure means the compressor is not being unloaded. A failed governor is the usual cause, and the safety valve is acting as the designed backup.

Question 36Air Brake System Components and Their Functions

What is the function of the double check valve used in a tractor protection or spring brake control circuit?

  1. It allows the higher of two pressure signals to pass while blocking the lower one Correct answer.
  2. It splits supply air equally between the primary and secondary circuits
  3. It reduces application pressure to the front brakes
  4. It drains moisture from the line it is installed in

Explanation

A double check valve has two inlets and one outlet and shifts to whichever inlet has the greater pressure. This lets a control signal reach the same component from either of two sources while preventing backflow to the other source.

Question 37Air Brake System Components and Their Functions

What is the purpose of the tractor protection valve's automatic function when the trailer air supply is lost?

  1. It closes the trailer supply line and applies the trailer spring brakes Correct answer.
  2. It increases application pressure to the trailer service brakes
  3. It bypasses the trailer relay valve to speed up release
  4. It vents the tractor's primary reservoir to atmosphere

Explanation

If the trailer separates or supply pressure drops, the tractor protection valve closes the trailer supply line to keep tractor air from escaping. With supply air lost, the trailer spring brakes apply automatically.

Question 38Air Brake System Components and Their Functions

A driver releases the parking brake valve and hears a steady leak at the dash valve that continues until the valve is pushed back in. What is the most likely cause?

  1. A stuck open reservoir drain valve
  2. A leaking service brake chamber diaphragm
  3. A defective parking brake control valve Correct answer.
  4. A failed low pressure warning switch

Explanation

A leak that appears only in one position of the parking control valve points to the valve itself not sealing internally. A service chamber or drain valve leak would be present regardless of parking valve position.

Question 39Air Brake System Components and Their Functions

Why is the compressor discharge line often fitted with a coiled section or cooling arrangement before it reaches the supply reservoir?

  1. To help cool the hot compressed air so moisture condenses where it can be drained Correct answer.
  2. To reduce the pressure delivered to the governor
  3. To increase the compressor's displacement
  4. To store enough air to release the spring brakes

Explanation

Compressing air raises its temperature, and cooling it before storage causes water vapor to condense in the supply tank rather than downstream. That condensation can then be removed through the drain valve.

Question 40Air Brake System Components and Their Functions

During a brake application, the driver hears air exhausting continuously from the relay valve area and the trailer brakes apply weakly. Which explanation best fits these symptoms?

  1. The trailer spring brake chambers are fully released
  2. The compressor is building pressure too quickly
  3. The low pressure warning buzzer is sounding
  4. The relay valve is leaking internally or its exhaust port is stuck open Correct answer.

Explanation

A relay valve that leaks internally or has a stuck-open exhaust will lose application air before it reaches the chambers. That produces weak brake application and a continuous exhaust sound at the valve.

Question 41Dual Air Brake Systems and Safety Circuit Operation

On a dual air brake system, which circuit is normally charged first as pressure builds from zero?

  1. The secondary (rear brake) circuit
  2. The primary (rear brake) circuit Correct answer.
  3. The trailer supply line only
  4. Both circuits charge at exactly the same rate

Explanation

The primary circuit is the first to be charged when air pressure builds from zero, which is why the primary gauge rises before the secondary gauge on a typical dual-needle dash display. This sequence lets the rear brakes reach working pressure early. The secondary circuit follows once the primary reaches its cut-out range.

Question 42Dual Air Brake Systems and Safety Circuit Operation

What is the purpose of the dual air brake system design?

  1. To increase total reservoir capacity for long downgrades
  2. To provide a backup circuit if one circuit fails Correct answer.
  3. To reduce the number of gauges the driver must monitor
  4. To allow the trailer to brake independently of the tractor

Explanation

A dual system splits the brakes into two separate circuits so that a failure of one circuit still leaves a functioning circuit to stop the vehicle. The driver keeps a degree of braking even after a major leak or line rupture. This redundancy is the core safety reason the system exists.

Question 43Dual Air Brake Systems and Safety Circuit Operation

In a dual air brake system, which component prevents air from escaping through a ruptured line in one circuit?

  1. The double check valve Correct answer.
  2. The alcohol evaporator
  3. The low pressure warning switch
  4. The safety valve

Explanation

The double check valve lets air flow from whichever circuit has the higher pressure while blocking backflow, so a rupture in one circuit does not bleed the other circuit dry. It effectively isolates the failed side. This keeps the intact circuit available for braking.

Question 44Dual Air Brake Systems and Safety Circuit Operation

The low air pressure warning must activate before pressure drops below what value?

  1. 30 psi
  2. 80 psi
  3. 60 psi Correct answer.
  4. 45 psi

Explanation

Federal standards require the low pressure warning device to activate before air pressure falls below 60 psi. The warning gives the driver time to stop safely before spring brakes apply automatically. Below roughly 45 psi the spring brakes begin to engage on many vehicles.

Question 45Dual Air Brake Systems and Safety Circuit Operation

A tractor with a dual air brake system has a gauge showing normal pressure in the primary circuit but zero in the secondary. What should the driver do?

  1. Place the vehicle out of service until the secondary circuit is repaired Correct answer.
  2. Drive normally because one circuit is still charged
  3. Pump the brake pedal rapidly to force air into the secondary circuit
  4. Increase engine rpm to build secondary pressure

Explanation

A circuit reading zero means that half of the braking system is not functioning, so the vehicle must not be driven until it is repaired. Pumping or revving will not restore a circuit that has lost pressure through a leak or failed component. The safe choice is to tag the vehicle out of service.

Question 46Dual Air Brake Systems and Safety Circuit Operation

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

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

Explanation

The safety valve is a spring-loaded relief device that opens if reservoir pressure rises above its setting, protecting the tank from over-pressurization. It is a backup to the governor in case the compressor does not cut out. Normal operation keeps it closed.

Question 47Dual Air Brake Systems and Safety Circuit Operation

The governor on an air compressor serves which purpose?

  1. It controls when the compressor loads and unloads to maintain reservoir pressure Correct answer.
  2. It meters air to the trailer supply line
  3. It limits how fast the spring brakes can release
  4. It regulates the pressure applied to the brake chambers

Explanation

The governor senses reservoir pressure and loads or unloads the compressor to keep pressure within the cut-in and cut-out range. When the upper limit is reached it unloads the compressor so it stops pumping. When pressure falls to the cut-in point it loads the compressor again.

Question 48Dual Air Brake Systems and Safety Circuit Operation

During a pre-trip check, the compressor should build pressure from 85 to 100 psi within what time?

  1. About 3 minutes
  2. About 45 seconds Correct answer.
  3. About 10 minutes
  4. There is no time standard

Explanation

A common CDL pre-trip standard is that the compressor should build from 85 to 100 psi in about 45 seconds at normal governed engine speed. This confirms the compressor and governor are working properly. A much slower buildup suggests a worn compressor or a leak.

Question 49Dual Air Brake Systems and Safety Circuit Operation

When you release the parking brake control on a vehicle with spring brakes, what actually happens?

  1. Air pressure is applied to the service side of the chamber
  2. Air pressure is applied to the spring side of the chamber
  3. The spring is mechanically latched open by a cable
  4. Air pressure is released from the spring side of the chamber Correct answer.

Explanation

Spring brakes are applied by heavy springs and released by air pressure pushing against them. Releasing the parking brake control sends air into the spring (parking) chamber, compressing the spring and freeing the brakes. Losing that air causes the spring to apply the brakes automatically.

Question 50Dual Air Brake Systems and Safety Circuit Operation

What is the purpose of the tractor protection valve?

  1. It balances tractor and trailer reservoir pressure at all times
  2. It limits the top speed of the tractor
  3. It automatically applies the trailer spring brakes when the engine starts
  4. It prevents the tractor from losing air if the trailer breaks away or its lines rupture Correct answer.

Explanation

The tractor protection valve closes the tractor-to-trailer air lines when trailer pressure drops too low or the lines separate, keeping the tractor's air supply intact. It also signals the trailer emergency system to apply the trailer brakes. This protects the tractor's ability to stop itself.