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ASE H4 Brakes (Transit Bus) study materials
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ASE H4 Brakes (Transit Bus) practice questions

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Question 1Air Brake System Components and Operation

On a typical transit bus air brake system, what is the primary function of the supply reservoir?

  1. Stores air for the service brake circuit only
  2. Holds air that has already passed through the brake chambers
  3. Stores air at full system pressure for delivery to other circuits Correct answer.
  4. Regulates the maximum system pressure at the compressor

Explanation

The supply (wet) reservoir receives air directly from the compressor and stores it at system pressure for distribution to the other circuits. It is the first tank in the system and is not dedicated to one brake function. Pressure regulation is handled by the governor, not the tank.

Question 2Air Brake System Components and Operation

What component senses reservoir pressure and controls the compressor's loading and unloading cycles?

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

Explanation

The governor monitors reservoir pressure and signals the compressor to load or unload to maintain the designed operating range. It cuts the compressor in at the low setting and out at the high setting. The other listed valves serve different functions.

Question 3Air Brake System Components and Operation

A bus air brake system has a compressor cut-out pressure of 130 psi and cut-in of 105 psi. What does the cut-in pressure represent?

  1. The pressure at which the compressor begins pumping again Correct answer.
  2. The pressure at which the compressor stops pumping
  3. The pressure at which the low-pressure warning activates
  4. The pressure at which the safety valve opens

Explanation

Cut-in is the lower governor setting where the compressor resumes pumping to restore reservoir pressure. Cut-out is the upper setting where the compressor stops. The warning buzzer and safety valve are separate devices with their own thresholds.

Question 4Air Brake System Components and Operation

What is the purpose of the alcohol evaporator sometimes fitted in a bus air system?

  1. To raise the dew point of the air in the reservoirs
  2. To increase the compressor's discharge pressure
  3. To introduce alcohol vapor that prevents moisture from freezing in valves Correct answer.
  4. To lubricate the compressor's internal moving parts

Explanation

An alcohol evaporator adds alcohol vapor to the air stream so that any condensed moisture is less likely to freeze and immobilize valves in cold weather. It does not lubricate the compressor or change pressure. Air dryers are the more modern approach to moisture control.

Question 5Air Brake System Components and Operation

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

  1. It limits the pressure delivered to the brake chambers to prevent wheel lockup
  2. It closes off a non-critical circuit when pressure falls so brake circuits retain air Correct answer.
  3. It releases excess pressure from the supply reservoir to atmosphere
  4. It equalizes pressure between the primary and secondary circuits

Explanation

A pressure protection valve isolates auxiliary circuits (such as doors or suspension) when system pressure drops below a set value, preserving remaining air for the brake circuits. It protects braking capability rather than controlling brake application pressure.

Question 6Air Brake System Components and Operation

In a dual-circuit (primary/secondary) air brake system, how are the two circuits normally arranged?

  1. One circuit serves all brakes and the other serves only the parking brake
  2. Both circuits feed the same chambers in parallel at all times
  3. Each circuit serves half the brake chambers on every axle
  4. Front and rear axles are split so each circuit serves one axle group Correct answer.

Explanation

A common split is front axle on one circuit and rear axle(s) on the other, so a single failure leaves one set of service brakes operational. The exact split varies by manufacturer, but the principle is to isolate a failure to one axle group.

Question 7Air Brake System Components and Operation

What is the purpose of the supply (wet) reservoir being located upstream of the other reservoirs?

  1. It cools the air and allows moisture and oil to condense before air reaches the other tanks Correct answer.
  2. It provides a reserve for the low-pressure warning system
  3. It meters air to the foot valve
  4. It stores the air used for the parking brake release

Explanation

The supply reservoir acts as a settling and condensing chamber, collecting moisture and oil carryover so downstream tanks receive cleaner air. This is why it is drained frequently. It does not meter air or serve the warning system.

Question 8Air Brake System Components and Operation

A bus operator reports that the low-pressure warning buzzer sounds briefly each time the compressor unloads. Which condition best explains this?

  1. The warning switch setting is above the governor cut-in pressure Correct answer.
  2. The safety valve is leaking
  3. The compressor is worn and cannot build pressure
  4. The governor cut-out is set too high

Explanation

If the warning switch trips at a pressure higher than cut-in, the buzzer will sound during the normal drop between cut-out and cut-in. The warning should be set below cut-in so it only alerts on a genuine loss of pressure. A high cut-out or worn compressor would not produce a buzz on each unload cycle.

Question 9Air Brake System Components and Operation

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

  1. It releases air from the brake chambers when the pedal is released
  2. It meters air pressure to the brake chambers in proportion to pedal travel Correct answer.
  3. It sets the governor cut-out pressure
  4. It applies the parking brake when the pedal is depressed

Explanation

The foot valve is the driver's service brake control; it modulates delivered pressure in proportion to how far the pedal is pushed. On release it exhausts the delivered air so the brakes release. Parking brake application is a separate control.

Question 10Air Brake System Components and Operation

What is the function of a relay valve in a bus air brake system?

  1. It prevents the spring brake from applying when air pressure is low
  2. It reduces reservoir pressure to a fixed value for the brake chambers
  3. It speeds application and release of air at the brake chambers using a nearby reservoir supply Correct answer.
  4. It automatically applies the brakes if the driver fails to act

Explanation

A relay valve uses a small control signal from the foot valve to admit air from a nearby reservoir directly to the brake chambers. This shortens the distance air must travel, improving application and release timing. It does not set a fixed pressure or apply brakes automatically.

Question 11Air Brake System Components and Operation

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

  1. To shorten the air path to the rear brake chambers and improve timing Correct answer.
  2. To keep it away from engine heat
  3. To reduce the number of reservoirs required
  4. To allow it to serve as the low-pressure warning sensor

Explanation

Mounting the relay valve close to the chambers it serves minimizes the volume of air that must travel from the control valve, giving faster and more balanced brake response. Heat and warning sensing are not the reasons.

Question 12Air Brake System Components and Operation

What is the purpose of a quick-release valve?

  1. It dumps air from the supply tank when pressure is too high
  2. It releases the spring brake in an emergency
  3. It quickly fills the reservoir after a brake application
  4. It rapidly exhausts air from a brake chamber to speed brake release Correct answer.

Explanation

A quick-release valve is placed near the brake chambers to exhaust release air locally rather than sending it all the way back through the foot valve. This shortens release time. It does not fill reservoirs or release spring brakes.

Question 13Air Brake System Components and Operation

A spring brake (parking/emergency brake) chamber contains two sections. What applies the parking brake?

  1. Air pressure in the service chamber
  2. A heavy mechanical spring when air is exhausted from the spring cavity Correct answer.
  3. A cable operated by the driver's hand lever
  4. Air pressure admitted to the spring cavity

Explanation

The spring brake uses a large spring to apply the brake when air is released from the spring cavity; air pressure compresses the spring to release the brake. This makes it fail-safe: loss of air applies the brake. The service chamber is separate and handles normal braking.

Question 14Air Brake System Components and Operation

What is the purpose of the inversion (spring brake priority) valve in a bus air system?

  1. It reverses the direction of air flow in the supply line
  2. It ensures the spring brakes are not released until sufficient air pressure is available Correct answer.
  3. It inverts the signal from the foot valve to apply the brakes
  4. It converts air pressure to an electrical signal

Explanation

The inversion valve is used with spring brakes so that the parking brake cannot be released unless system pressure is adequate, and it can apply the spring brakes automatically in some failure modes. It does not invert a service signal or convert pressure to electricity.

Question 15Air Brake System Components and Operation

What does the parking brake (push-pull) control valve do when the driver pulls the knob out?

  1. It closes the supply reservoir to the rest of the system
  2. It supplies air to the service chambers to hold the brakes
  3. It supplies air to the spring cavities to apply the brakes
  4. It exhausts air from the spring cavities so the springs apply the brakes Correct answer.

Explanation

Pulling the parking brake knob exhausts air from the spring brake cavities, allowing the springs to apply the brakes. Pushing the knob in admits air to compress the springs and release the brakes. Service chambers are not involved in parking.

Question 16Air Brake System Components and Operation

A bus with spring brakes has lost all air pressure. What is the state of the parking brakes?

  1. They are applied because the springs are no longer held off by air Correct answer.
  2. They are released because no air is available to hold them off
  3. They are applied only if the driver pulls the knob
  4. They are unaffected because they are hydraulically assisted

Explanation

Spring brakes are fail-safe: air pressure holds the springs compressed to keep the brakes released, so a total loss of air lets the springs apply the brakes. This is why a bus with no air cannot be towed without caging the springs.

Question 17Air Brake System Components and Operation

What is the purpose of caging a spring brake chamber?

  1. To increase spring force for a stronger parking brake
  2. To mechanically compress and hold the spring so the brake is released for towing or service Correct answer.
  3. To connect the chamber to the quick-release valve
  4. To lock the service chamber in the applied position

Explanation

Caging uses a mechanical bolt to compress the spring and hold the brake released when air cannot be supplied, such as for towing or brake work. It is a manual override of the spring, not a way to increase force.

Question 18Air Brake System Components and Operation

What is the function of an automatic slack adjuster?

  1. It automatically applies the parking brake when the bus stops
  2. It maintains correct brake chamber pushrod stroke as linings wear Correct answer.
  3. It adjusts the governor cut-out pressure
  4. It bleeds moisture from the reservoir automatically

Explanation

An automatic slack adjuster takes up clearance as linings wear so pushrod stroke stays within specification. Correct stroke is essential for proper braking and for the spring brake to release fully. It does not control parking or system pressure.

Question 19Air Brake System Components and Operation

A technician measures a brake chamber pushrod stroke that exceeds the adjustment limit. What is the most likely consequence?

  1. The compressor will cycle more frequently
  2. The brakes will apply too quickly and lock the wheels
  3. The low-pressure warning will sound continuously
  4. Braking force will be reduced and the spring brake may not release fully Correct answer.

Explanation

Excessive pushrod stroke means the chamber uses much of its travel before the shoes contact the drum, reducing available braking force. In spring brake chambers, excessive stroke can also prevent full release. It does not cause lockup or affect compressor cycling.

Question 20Air Brake System Components and Operation

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

  1. To remove moisture and contaminants from the compressed air Correct answer.
  2. To lubricate the air valves
  3. To cool the compressed air before it enters the reservoir
  4. To increase the pressure delivered to the brake chambers

Explanation

An air dryer removes water vapor and oil/particulate contamination before air reaches the reservoirs, reducing freeze-ups and corrosion. It replaces or supplements the alcohol evaporator. It does not change pressure or lubricate valves.

Question 21Air Brake System Components and Operation

During the compressor unloaded (cut-out) phase, what is the compressor doing?

  1. Circulating air without building reservoir pressure Correct answer.
  2. Pumping air into the supply reservoir at full output
  3. Shutting off completely and not turning
  4. Releasing air through the safety valve

Explanation

When the governor unloads the compressor, the compressor keeps turning but its output is directed so it does not build reservoir pressure. This reduces load and wear. It does not stop turning, and the safety valve is not involved.

Question 22Air Brake System Components and Operation

A bus is slow to build air pressure from zero. Which condition is the least likely cause?

  1. A worn compressor
  2. A restricted compressor intake
  3. A governor cut-out set too high Correct answer.
  4. A leaking discharge line between compressor and reservoir

Explanation

A high cut-out setting changes when the compressor stops, not how fast it builds pressure from zero. Restricted intake, discharge leaks, and a worn compressor all reduce the rate of pressure rise. Build time is a key diagnostic for compressor and leak problems.

Question 23Air Brake System Components and Operation

What is the function of the safety valve on the supply reservoir?

  1. It releases air to atmosphere if reservoir pressure exceeds a set limit Correct answer.
  2. It drains condensate from the reservoir
  3. It maintains a minimum pressure in the reservoir
  4. It signals the governor to load the compressor

Explanation

The safety valve is a pressure-relief device that opens if pressure rises above its setting, protecting the system if the governor fails. It does not maintain minimum pressure or signal the governor. Manual drains handle condensate.

Question 24Air Brake System Components and Operation

What is the role of a check valve between the compressor and the supply reservoir?

  1. It removes oil from the compressed air
  2. It prevents air from flowing back from the reservoir to the compressor Correct answer.
  3. It equalizes pressure between the two service circuits
  4. It limits the maximum pressure the compressor can produce

Explanation

A check valve permits air to flow toward the reservoir but blocks reverse flow, so reservoir air does not bleed back through the compressor when it unloads. Pressure limiting is the governor's role, and oil removal is the dryer's job.

Question 25Air Brake System Components and Operation

A bus has good pressure buildup, but the low-pressure warning stays on after the system reaches normal pressure. Which is the most likely cause?

  1. The warning switch or its circuit is faulty Correct answer.
  2. The safety valve is stuck open
  3. The compressor is worn
  4. The governor cut-out is too low

Explanation

If pressure builds normally but the warning persists, the pressure switch or its wiring is the prime suspect, since the warning should clear once pressure exceeds the switch threshold. A low cut-out or worn compressor would show low pressure on the gauge, and a stuck safety valve would vent air.

Question 26Air Brake System Components and Operation

Which component in a transit bus air brake system removes moisture and contaminants from the compressed air before it reaches the reservoirs?

  1. The governor
  2. The alcohol evaporator
  3. The air dryer Correct answer.
  4. The low-pressure indicator

Explanation

The air dryer uses a desiccant bed to strip water vapor and oil carryover from compressor discharge air, then periodically purges the collected moisture. Locating it between the compressor and the supply reservoir protects downstream components from corrosion and freeze-up.

Question 27Air Brake System Components and Operation

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

  1. It regulates compressor loading and unloading at preset pressure levels Correct answer.
  2. It meters air flow to the trailer supply line
  3. It proportions front-to-rear brake pressure during a stop
  4. It limits maximum vehicle speed when the parking brake is released

Explanation

The governor monitors reservoir pressure and, through a signal line to the compressor unloader, starts and stops compressor pumping between cut-in and cut-out pressures. This keeps system pressure within the designed operating range.

Question 28Air Brake System Components and Operation

A bus is equipped with a manifold-drain (ping tank) arrangement. What is the purpose of this component?

  1. It provides a mounting point for the compressor discharge check valve
  2. It collects oil and water that condense between the compressor and the supply reservoir Correct answer.
  3. It smooths pressure pulsations to the low-pressure warning switch
  4. It stores reserve air for the parking brake release circuit

Explanation

A manifold drain tank gives compressor discharge air a place to slow down so entrained oil and water can condense and be drained off before the air reaches the wet tank. Removing these contaminants early extends the life of valves and the dryer.

Question 29Air Brake System Components and Operation

What does the application valve (brake valve) deliver to a relay valve when the driver presses the treadle?

  1. A control signal pressure proportional to treadle force Correct answer.
  2. A fixed pilot pressure set by the governor
  3. Full reservoir pressure regardless of treadle position
  4. A vacuum signal to the relay piston

Explanation

The foot valve acts as a regulating control valve, sending a small, graduated signal pressure to the relay valve that is proportional to how far the treadle is applied. The relay valve then delivers reservoir air at a corresponding pressure to the brake chambers.

Question 30Air Brake System Components and Operation

What is the function of a two-way check valve installed between the primary and secondary circuits feeding a relay valve?

  1. It blocks both circuits until pressure equalizes
  2. It vents excess pressure to atmosphere
  3. It divides supply air equally between both circuits
  4. It allows whichever circuit has higher pressure to supply the relay valve Correct answer.

Explanation

A two-way check valve passes the higher of two inlet pressures and seals off the lower one, so a single device can be fed by either circuit. This lets the relay valve continue to function if one circuit loses pressure.

Question 31Air Brake System Components and Operation

What does the brake chamber pushrod do when air pressure is applied to the service side of a spring brake chamber?

  1. It opens the inversion valve exhaust port
  2. It extends and rotates the slack adjuster to apply the brakes Correct answer.
  3. It retracts and releases the parking spring
  4. It resets the governor to cut-in pressure

Explanation

Service air acting on the diaphragm moves the pushrod outward, rotating the slack adjuster and camshaft to apply the foundation brakes. Releasing the treadle lets the air exhaust and the return spring retract the pushrod.

Question 32Air Brake System Components and Operation

A bus has a spring brake chamber with a failed service diaphragm but intact spring and spring-side components. What is the condition of the parking brake?

  1. The parking brake releases but cannot be applied
  2. The parking brake applies but cannot be released
  3. The parking brake cannot be applied
  4. The parking brake still applies and releases normally Correct answer.

Explanation

The service diaphragm and the spring-side piston/spring operate independently within the chamber. A service-side failure affects only service braking, so parking brake apply and release remain functional.

Question 33Air Brake System Components and Operation

What is the purpose of the tractor protection valve equivalent found on some transit buses, the park brake control valve's supply check function?

  1. It automatically applies the parking brake at cut-out pressure
  2. It prevents loss of the remaining air supply through an open parking brake circuit Correct answer.
  3. It provides a fast idle signal to the engine during parking
  4. It regulates parking brake application force to a fixed value

Explanation

A check valve in the parking brake supply path keeps air in the spring brake circuit from escaping back through a leaking or disconnected control line. This allows the spring brakes to be released and held off even if the control circuit is compromised.

Question 34Air Brake System Components and Operation

What is the purpose of the compressor discharge line being routed with a downward slope or loop?

  1. To increase compressor output pressure
  2. To provide a mounting point for the governor
  3. To encourage condensed oil and water to drain away from the compressor Correct answer.
  4. To reduce compressor noise

Explanation

Sloping or looping the discharge line lets condensed liquids collect and drain rather than run back into the compressor or forward into the dryer. Liquid carryover can damage compressor valves and saturate the desiccant.

Question 35Air Brake System Components and Operation

What is the function of the service reservoir check valves in a multi-reservoir system?

  1. They isolate each reservoir so a loss in one circuit does not drain the others Correct answer.
  2. They vent the reservoirs when the engine is shut down
  3. They regulate the pressure delivered to the brake chambers
  4. They allow reservoirs to be filled only from the secondary circuit

Explanation

Check valves at the inlet of each reservoir allow air in but prevent it from flowing back out, so a failure in one circuit cannot deplete pressure in the others. This preserves braking capability in the remaining circuit.

Question 36Air Brake System Components and Operation

A technician finds the compressor running continuously and system pressure climbing past the cut-out point. Which component is the most likely cause?

  1. A leaking service diaphragm
  2. A failed quick-release valve
  3. A governor that is not sending an unload signal Correct answer.
  4. A stuck-open drain cock

Explanation

Continuous pumping past cut-out indicates the compressor is not being unloaded, which is the governor's job. If the governor fails to signal the unloader, the compressor keeps building pressure until the safety valve relieves it.

Question 37Air Brake System Components and Operation

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

  1. It activates a visual and audible warning when system pressure falls below a safe level Correct answer.
  2. It illuminates the brake lights during a stop
  3. It applies the spring brakes automatically at a preset pressure
  4. It disables the compressor to prevent overpressure

Explanation

The low-pressure switch senses reservoir pressure and triggers a warning lamp and buzzer when pressure drops below the safe operating threshold. This alerts the driver before braking capability is compromised.

Question 38Air Brake System Components and Operation

How does a relay valve reduce brake application time on a bus with long air lines to the rear axle?

  1. It increases the control signal pressure to the brake chambers
  2. It stores air in the brake chamber between stops
  3. It delivers reservoir air locally at the axle instead of waiting for air to travel from the foot valve Correct answer.
  4. It bypasses the governor during braking

Explanation

A relay valve uses a small control signal from the foot valve to open a local reservoir supply right at the axle, so the large volume of air needed for application travels only a short distance. This shortens lag time and improves brake response.

Question 39Air Brake System Components and Operation

What is the purpose of the inversion valve in a system where spring brakes are used for emergency braking on a split-circuit bus?

  1. It applies the spring brakes when the service circuit pressure drops below a set value Correct answer.
  2. It reverses the polarity of the warning switch
  3. It inverts the direction of pushrod travel
  4. It converts spring brake air to service brake air

Explanation

The inversion valve senses a loss of pressure in the service circuit and, when pressure falls below its setting, delivers air to release the spring brakes only if the parking control is in the released position; when the control is applied or supply is lost, it lets the spring brakes apply. This provides modulated emergency braking.

Question 40Air Brake System Components and Operation

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

  1. It introduces alcohol vapor to prevent moisture in the system from freezing Correct answer.
  2. It removes oil from the compressor discharge air
  3. It lubricates the brake chamber diaphragms
  4. It increases the compressor cut-out pressure

Explanation

An alcohol evaporator adds a small amount of alcohol vapor to the air stream so any condensed water in lines and valves is less likely to freeze in cold weather. It is a freeze-protection device, not a filter.

Question 41Air Supply and Compressor Systems

On a typical transit bus air brake system, the compressor is driven by which of the following?

  1. A hydraulic motor from the power steering pump
  2. An electric motor mounted on the firewall
  3. A dedicated air turbine using stored reservoir pressure
  4. A belt or gear drive off the engine Correct answer.

Explanation

The compressor must run whenever the engine runs so it can recharge the system continuously, so it is driven mechanically from the engine by belt, gear, or direct coupling. Electric or hydraulic drives are not the standard arrangement on transit coaches.

Question 42Air Supply and Compressor Systems

What is the primary function of the air dryer in a transit bus air system?

  1. To regulate the rate of compressor unloading
  2. To remove moisture and contaminants from compressed air Correct answer.
  3. To raise system pressure above governor cut-out
  4. To store a reserve of air for the brakes

Explanation

The air dryer uses a desiccant bed to absorb water vapor and trap oil and dirt before air reaches the wet tank. Removing moisture prevents corrosion and winter freeze-up in valves and lines.

Question 43Air Supply and Compressor Systems

A bus with a single-circuit charging system shows the compressor building pressure very slowly and the dash gauge climbs only at high engine RPM. Which condition best explains this?

  1. Leaking compressor discharge valve or worn compressor Correct answer.
  2. Governor cut-out pressure set too low
  3. Air dryer purge valve stuck closed
  4. Defective low-pressure indicator switch

Explanation

Slow pressure buildup that improves with engine speed points to reduced compressor output, such as worn rings, a leaking discharge valve, or a restricted intake. Governor and switch faults affect cut-out or warning points, not the rate of climb.

Question 44Air Supply and Compressor Systems

The governor on a transit bus air system serves which purpose?

  1. It limits maximum reservoir pressure by unloading the compressor at cut-out and reloading at cut-in Correct answer.
  2. It limits vehicle speed when air pressure is low
  3. It meters air flow to the brake chambers during a stop
  4. It automatically drains condensate from the wet tank

Explanation

The governor senses reservoir pressure and signals the compressor unloader to stop pumping at cut-out, then resumes at cut-in. This keeps pressure within the designed operating band.

Question 45Air Supply and Compressor Systems

During the compressor unloaded (cut-out) phase, what is happening inside the compressor?

  1. The compressor runs in reverse to relieve pressure
  2. The discharge line is vented to atmosphere continuously
  3. The compressor stops rotating until pressure drops
  4. The inlet valves are held open so air is not compressed into the system Correct answer.

Explanation

Unloading holds the intake valves open so the pistons move air back and forth without building pressure, which reduces load and heat while the compressor keeps turning. The drive remains engaged the whole time.

Question 46Air Supply and Compressor Systems

Technician A says the wet tank collects moisture and oil carryover before air moves downstream. Technician B says the wet tank is the reservoir that directly supplies the brake treadle valve. Who is correct?

  1. B only
  2. Both A and B
  3. Neither A nor B
  4. A only Correct answer.

Explanation

The wet tank receives air directly from the compressor and dryer and acts as a collection point for condensate and carryover. Brake delivery is normally taken from the dry tanks downstream, not the wet tank.

Question 47Air Supply and Compressor Systems

A transit bus is equipped with an automatic drain valve on the wet tank. The valve is found stuck in the open position. What is the most likely result?

  1. The compressor will overheat and seize
  2. The low-pressure warning will be permanently disabled
  3. System pressure will not build or will build very slowly Correct answer.
  4. The air dryer desiccant will become saturated instantly

Explanation

A drain valve stuck open creates a continuous leak to atmosphere, so the compressor cannot keep up and pressure stays low or never reaches cut-out. The governor will keep the compressor loaded, but output is lost through the open drain.

Question 48Air Supply and Compressor Systems

What is the purpose of the safety relief valve installed in the air system?

  1. To release air if governor control is lost and pressure exceeds a safe limit Correct answer.
  2. To balance pressure between the front and rear service circuits
  3. To purge moisture from the dryer during each compressor cycle
  4. To provide a quick release of air when the park brake is applied

Explanation

The safety valve is a pressure-limiting device that opens if reservoir pressure rises above its setting, protecting the system if the governor fails to unload the compressor. It is a backup, not a normal operating control.

Question 49Air Supply and Compressor Systems

A bus driver reports that the low-pressure warning buzzer sounds briefly each time the compressor reaches cut-out. Which condition is the most probable cause?

  1. Pressure gauge sending unit calibration
  2. Governor cut-out set too high
  3. A pressure switch with too narrow a differential or a wiring fault Correct answer.
  4. Defective compressor discharge valve

Explanation

A warning that appears only at cut-out suggests the switch is reacting to the pressure drop at the top of the cycle, often from a switch with an incorrect setting or a poor connection. Mechanical compressor faults would affect charging rate rather than trigger warnings at cut-out.

Question 50Air Supply and Compressor Systems

Which component prevents air from bleeding back from the reservoirs to the compressor when the compressor unloads?

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

Explanation

A check valve at the compressor discharge or dryer inlet holds reservoir pressure when the compressor is unloaded or stopped. Without it, air would escape back through the compressor.