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ASE S4 Brakes (School Bus) study materials
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ASE S4 Brakes (School Bus) practice questions

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Question 1Air Brake System Diagnosis and Repair

A school bus air brake system has a pressure gauge that is rising very slowly during build-up. Which component is most likely causing this condition?

  1. A leaking governor
  2. A stuck open drain valve
  3. A faulty alcohol evaporator
  4. A restricted compressor intake Correct answer.

Explanation

A restricted compressor intake reduces the volume of air the compressor can draw in, slowing pressure build-up. Leaking governor or open drain valves would typically prevent build-up entirely or cause excessive cycling, not a slow steady rise.

Question 2Air Brake System Diagnosis and Repair

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

  1. To regulate the application rate of the brakes
  2. To lubricate the compressor pistons
  3. To remove moisture and contaminants from the compressed air Correct answer.
  4. To increase the system pressure beyond governor cut-out

Explanation

The air dryer removes water vapor and oil/particulate contaminants from compressed air before it reaches the reservoirs. It does not increase pressure, lubricate the compressor, or control brake application rate.

Question 3Air Brake System Diagnosis and Repair

During a static leak test, the air pressure in a school bus drops from 120 psi to 100 psi in 3 minutes with the engine off and brakes released. What does this indicate?

  1. The low-pressure warning switch is faulty
  2. There is an excessive air leak in the system Correct answer.
  3. The governor cut-out pressure is set too high
  4. The compressor is not building pressure

Explanation

A pressure drop of more than 2 psi per minute (or the manufacturer's specification) with brakes released indicates a leak. A 20 psi drop in 3 minutes far exceeds allowable leakage and signals a system leak.

Question 4Air Brake System Diagnosis and Repair

A school bus has a dual air brake system. The rear service brake circuit loses pressure. What should happen to the front service brakes?

  1. They should be disabled to prevent skidding
  2. They should apply automatically and stay applied
  3. They should continue to operate normally Correct answer.
  4. They should also lose pressure immediately

Explanation

In a dual-circuit system, the circuits are independent so that a failure in one does not affect the other. The front service brakes should continue to function normally if the rear circuit fails.

Question 5Air Brake System Diagnosis and Repair

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

  1. To limit engine speed during compression
  2. To regulate the amount of air delivered to the brake chambers
  3. To automatically drain moisture from the reservoirs
  4. To control the compressor's loading and unloading to maintain reservoir pressure Correct answer.

Explanation

The governor senses reservoir pressure and signals the compressor to load (build pressure) or unload (stop building) at preset cut-in and cut-out pressures. It does not control engine speed, brake chamber delivery, or drain moisture.

Question 6Air Brake System Diagnosis and Repair

A school bus equipped with automatic slack adjusters has a brake chamber pushrod stroke that exceeds the adjustment limit. What is the most likely cause?

  1. The governor cut-out pressure is too low
  2. The brake chamber is too small for the application
  3. The slack adjuster is functioning correctly and no action is needed
  4. The automatic slack adjuster is failing to adjust properly Correct answer.

Explanation

If an automatic slack adjuster is working correctly, it should keep the pushrod stroke within limits. A stroke exceeding the limit indicates the adjuster is not adjusting properly and requires inspection or repair.

Question 7Air Brake System Diagnosis and Repair

What is the purpose of the safety valve in an air brake reservoir?

  1. To release air if system pressure exceeds a preset maximum Correct answer.
  2. To supply air to the brake chambers during a failure
  3. To allow manual draining of the reservoir
  4. To prevent air from flowing back to the compressor

Explanation

The safety valve is a pressure relief device that opens if reservoir pressure exceeds a safe preset limit, protecting the system from over-pressurization. It is not a drain, check valve, or emergency supply.

Question 8Air Brake System Diagnosis and Repair

A school bus air brake system has a compressor that runs continuously and the pressure gauge shows 150 psi and rising. Which component is most likely at fault?

  1. The air dryer purge valve
  2. The treadle valve
  3. The governor Correct answer.
  4. The relay valve

Explanation

The governor is responsible for unloading the compressor at cut-out pressure. If it fails to signal unloading, the compressor continues to run and pressure can rise above normal, potentially to the safety valve setting.

Question 9Air Brake System Diagnosis and Repair

What is the correct method to test the low-pressure warning device on a school bus air brake system?

  1. Apply the brakes and listen for an audible click
  2. Build pressure to governor cut-out, then reduce pressure and verify the warning activates before 60 psi Correct answer.
  3. Disconnect the warning switch and observe the gauge
  4. Turn on the engine and check that the warning activates above 100 psi

Explanation

The low-pressure warning must activate before pressure drops below a safe level, typically around 60 psi. The proper test is to build pressure, then bleed it down and confirm the warning activates at the specified pressure.

Question 10Air Brake System Diagnosis and Repair

A school bus has spring brakes. What happens when air pressure in the spring brake circuit drops below the spring brake actuation range?

  1. The spring brakes release
  2. The spring brakes remain in their current state
  3. The spring brakes apply gradually Correct answer.
  4. The service brakes apply fully

Explanation

Spring brakes are held off by air pressure. As pressure drops below the actuation range, the springs apply the brakes gradually, providing emergency stopping and parking function.

Question 11Air Brake System Diagnosis and Repair

What is the purpose of the inversion valve in a school bus air brake system?

  1. To invert the air flow direction during compressor unloading
  2. To prevent air from entering the spring brake chambers
  3. To allow a modulated spring brake application if the service brake circuit fails Correct answer.
  4. To automatically drain the air dryer

Explanation

The inversion valve provides a way to modulate the spring brakes (apply them gradually) in the event of a service brake circuit failure, allowing controlled stopping. It does not reverse air flow, block spring brake air, or drain the dryer.

Question 12Air Brake System Diagnosis and Repair

A school bus air brake system fails the compressor build-up test. The pressure rises from 85 to 100 psi in 60 seconds. What is the most likely cause?

  1. A misadjusted slack adjuster
  2. A leak in the service brake chambers
  3. A faulty low-pressure warning switch
  4. A worn compressor or drive belt Correct answer.

Explanation

The compressor build-up test measures how quickly the compressor can raise pressure. A slow build-up indicates the compressor is not pumping efficiently, often due to wear or a slipping drive belt.

Question 13Air Brake System Diagnosis and Repair

What is the function of the quick-release valve in an air brake system?

  1. To allow rapid draining of moisture from the system
  2. To release air from the reservoir when pressure is too high
  3. To quickly apply the brakes in an emergency
  4. To speed up the release of air from brake chambers when the brake is released Correct answer.

Explanation

The quick-release valve is located near the brake chambers and exhausts air quickly when the treadle valve is released, speeding up brake release. It does not apply brakes, relieve reservoir pressure, or drain moisture.

Question 14Air Brake System Diagnosis and Repair

A school bus has a brake pedal that feels spongy and has excessive travel. Which of the following is the most likely cause?

  1. A faulty governor
  2. Air in the hydraulic brake system Correct answer.
  3. A leaking air compressor
  4. A stuck open drain valve

Explanation

A spongy pedal with excessive travel is a classic symptom of air in the hydraulic brake system, which is compressible and causes a soft pedal. The other options relate to the air supply system, not the hydraulic brake circuit.

Question 15Air Brake System Diagnosis and Repair

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

  1. To prevent moisture in the air lines from freezing Correct answer.
  2. To increase the octane rating of the fuel
  3. To clean the air dryer desiccant
  4. To lubricate the brake chamber diaphragms

Explanation

The alcohol evaporator introduces alcohol vapor into the air system to lower the freezing point of moisture, preventing ice formation in air lines and valves during cold weather. It does not affect fuel, lubricate diaphragms, or clean the dryer.

Question 16Air Brake System Diagnosis and Repair

During a brake application leak test on a school bus, the pressure drops 5 psi in 1 minute with the brakes fully applied. What does this indicate?

  1. The leak rate is within acceptable limits
  2. The governor cut-out pressure is too high
  3. The compressor is not building enough pressure
  4. There is a leak in the applied brake circuit Correct answer.

Explanation

A pressure drop of more than 3 psi per minute (or manufacturer's specification) with brakes applied indicates a leak in the applied circuit. A 5 psi drop in 1 minute exceeds typical allowable limits.

Question 17Air Brake System Diagnosis and Repair

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

  1. To relay electrical signals to the brake lights
  2. To automatically adjust slack adjusters
  3. To prevent air from flowing back to the compressor
  4. To reduce the time lag between treadle valve application and brake chamber response Correct answer.

Explanation

The relay valve uses a small control signal from the treadle valve to quickly supply a large volume of air to the brake chambers, reducing application and release time. It does not handle electrical signals, adjust slack, or act as a check valve.

Question 18Air Brake System Diagnosis and Repair

A school bus air brake system has a compressor that is not building pressure. Which of the following is the least likely cause?

  1. A faulty governor stuck in the unload position
  2. A broken compressor drive belt
  3. A leaking safety valve
  4. A misadjusted slack adjuster Correct answer.

Explanation

A misadjusted slack adjuster affects brake chamber pushrod stroke and brake performance, but does not prevent the compressor from building pressure. The other options directly affect the compressor's ability to build or hold pressure.

Question 19Air Brake System Diagnosis and Repair

What is the correct procedure for testing the spring brakes on a school bus with air brakes?

  1. With the engine running, disconnect the compressor and observe the gauge
  2. With the engine off, drain the air pressure and verify the spring brakes apply Correct answer.
  3. With the engine running, apply the service brake and check for air leaks
  4. With the engine off and air pressure at normal, apply the parking brake and check for movement

Explanation

To test spring brakes, build normal pressure, then drain the system (or reduce pressure below the spring brake actuation range) and verify that the spring brakes apply and hold the vehicle. This tests the emergency/parking function.

Question 20Air Brake System Diagnosis and Repair

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

  1. To increase the compressor output
  2. To automatically drain moisture
  3. To allow air flow in one direction only Correct answer.
  4. To regulate the pressure in the reservoir

Explanation

A check valve permits air to flow in one direction and prevents backflow, protecting the system from pressure loss. It does not regulate pressure, drain moisture, or increase compressor output.

Question 21Air Brake System Diagnosis and Repair

A school bus has a brake chamber with a pushrod stroke that is too short. What is the most likely effect on braking performance?

  1. The brakes may not fully apply, reducing stopping power Correct answer.
  2. The spring brakes will not release
  3. The brakes will drag and overheat
  4. The low-pressure warning will activate

Explanation

If the pushrod stroke is too short, the slack adjuster is too tight, and the brake shoes may not fully contact the drum, reducing braking force. Too short a stroke can also cause brake drag if the shoes are too close, but the primary concern is inadequate application.

Question 22Air Brake System Diagnosis and Repair

What is the function of the treadle valve in an air brake system?

  1. To control the amount of air pressure delivered to the brake chambers based on pedal force Correct answer.
  2. To prevent moisture from entering the system
  3. To regulate the compressor cut-in and cut-out pressures
  4. To automatically adjust the slack adjusters

Explanation

The treadle valve (foot valve) modulates air pressure to the brake chambers in proportion to the force applied to the pedal. It does not adjust slack, regulate compressor pressures, or remove moisture.

Question 23Air Brake System Diagnosis and Repair

During a school bus air brake inspection, a technician finds that the compressor builds pressure from 85 to 100 psi in 45 seconds. What does this indicate?

  1. The compressor is building pressure too slowly and may need repair
  2. The air dryer is not purging properly
  3. The governor cut-out pressure is set too low
  4. The compressor is functioning within normal specifications Correct answer.

Explanation

Typical specifications require the compressor to build from 85 to 100 psi within a certain time (often 45 seconds or less for a school bus). A 45-second build-up is generally within acceptable limits, indicating normal compressor performance.

Question 24Air Brake System Diagnosis and Repair

What is the purpose of the front brake limiting valve (if equipped) on a school bus?

  1. To reduce front brake pressure when the vehicle is empty to prevent wheel lockup Correct answer.
  2. To prevent the front brakes from operating in reverse
  3. To increase front brake pressure for better stopping
  4. To automatically apply the front brakes during a rear circuit failure

Explanation

A front brake limiting valve (often called a 'slippery road' valve) reduces front brake pressure under certain conditions to help prevent front wheel lockup on slippery surfaces or when lightly loaded. It does not increase pressure, apply brakes automatically, or disable reverse braking.

Question 25Air Brake System Diagnosis and Repair

A school bus air brake system has a pressure gauge that reads 0 psi, but the low-pressure warning buzzer is not sounding. What is the most likely cause?

  1. The governor is stuck in the load position
  2. The warning buzzer is faulty or disconnected Correct answer.
  3. The safety valve is stuck open
  4. The compressor is building too much pressure

Explanation

With 0 psi, the low-pressure warning should activate. If it does not, the warning device (buzzer or light) or its circuit is likely faulty. The other options would not explain the absence of a warning at zero pressure.

Question 26Air Brake System Diagnosis and Repair

On a school bus air brake system, what is the primary purpose of the air dryer?

  1. To increase the pressure output of the compressor
  2. To cool the air before it enters the reservoir
  3. To remove moisture and contaminants from the compressed air Correct answer.
  4. To regulate the pressure sent to the brake chambers

Explanation

The air dryer uses a desiccant to absorb water vapor and filters out oil and dirt, preventing moisture from reaching the reservoirs and brake components. It does not boost pressure, regulate brake application pressure, or primarily cool the air.

Question 27Air Brake System Diagnosis and Repair

A school bus air brake system has a governor that is malfunctioning. What is the most likely symptom?

  1. The low-pressure warning buzzer will sound continuously
  2. The compressor will not build pressure above 60 psi
  3. The spring brakes will apply automatically
  4. The compressor will not unload and pressure will rise excessively Correct answer.

Explanation

The governor controls when the compressor loads and unloads. If it fails to unload the compressor, pressure can rise above the cut-out point, potentially causing safety valve release or component damage. A failed governor does not directly cause low pressure, warning buzzer issues, or spring brake application.

Question 28Air Brake System Diagnosis and Repair

What is the function of the supply reservoir in a school bus air brake system?

  1. To automatically drain moisture from the system
  2. To regulate the pressure applied to the brake chambers
  3. To store air for the front brake circuit only
  4. To provide a reserve of compressed air for the braking system Correct answer.

Explanation

The supply reservoir (also called the wet tank) receives air from the compressor and stores it for distribution to the other reservoirs. It does not serve only the front circuit, regulate application pressure, or automatically drain moisture.

Question 29Air Brake System Diagnosis and Repair

During a static leak test on a school bus air brake system, the pressure drops 3 psi in one minute with the brakes released and the engine off. What does this indicate?

  1. The governor cut-in pressure is too low
  2. The leak rate is within acceptable limits Correct answer.
  3. The compressor is faulty
  4. There is a leak in the service brake circuit

Explanation

For a school bus, a pressure loss of less than 3 psi per minute with brakes released is generally acceptable. A higher drop would indicate a leak. The compressor and governor are not directly tested by this static leak test.

Question 30Air Brake System Diagnosis and Repair

A school bus has spring brakes. What happens when air pressure in the spring brake circuit drops below the spring brake application threshold?

  1. The spring brakes gradually apply and eventually lock the wheels Correct answer.
  2. The service brakes become more effective
  3. The spring brakes release completely
  4. The compressor automatically increases pressure

Explanation

Spring brakes are applied by heavy springs and released by air pressure. When air pressure drops below the threshold, the springs progressively apply the brakes, eventually locking the wheels to fail-safe the vehicle. They do not release; they apply.

Question 31Air Brake System Diagnosis and Repair

During a school bus air brake inspection, a technician finds that the compressor builds pressure from 85 to 100 psi in 45 seconds. What is the correct interpretation?

  1. The build-up rate cannot be determined without a loaded test
  2. The build-up rate is too fast and indicates a faulty governor
  3. The build-up rate is too slow and indicates a problem
  4. The build-up rate is acceptable Correct answer.

Explanation

For a school bus, the compressor should build from 85 to 100 psi within 45 seconds at fast idle. Meeting this time indicates acceptable performance. A longer time would be too slow; a shorter time is generally acceptable.

Question 32Air Brake System Diagnosis and Repair

A school bus air brake system has a pressure gauge that reads 0 psi, but the low-pressure warning buzzer is not sounding. Which of the following is the most likely cause?

  1. The warning buzzer or its circuit is faulty Correct answer.
  2. The compressor is working correctly
  3. The safety valve is stuck open
  4. The governor is functioning properly

Explanation

If pressure is 0 psi, the low-pressure warning should activate. If it does not, the warning device or its electrical circuit is likely faulty. The other options do not explain the lack of warning.

Question 33Air Brake System Diagnosis and Repair

What is the function of the pressure protection valve in a school bus air brake system?

  1. To isolate a failed circuit and preserve air pressure for other circuits Correct answer.
  2. To increase the pressure to the spring brakes
  3. To protect the compressor from over-pressurization
  4. To automatically drain the reservoirs

Explanation

The pressure protection valve closes off a circuit that has lost pressure, allowing the rest of the system to maintain pressure. It does not protect the compressor, drain reservoirs, or boost spring brake pressure.

Question 34Air Brake System Diagnosis and Repair

A school bus air brake system has a leak in the service brake circuit. Which test is most likely to reveal this leak?

  1. A slack adjuster stroke check
  2. A static leak test with the brakes applied Correct answer.
  3. A compressor build-up test
  4. A governor cut-out test

Explanation

A static leak test with the brakes applied pressurizes the service brake circuit and will show a pressure drop if there is a leak. The other tests do not specifically check for service circuit leaks.

Question 35Air Brake System Diagnosis and Repair

What is the purpose of the automatic slack adjuster on a school bus air brake system?

  1. To automatically adjust the compressor cut-out pressure
  2. To automatically maintain proper brake chamber pushrod stroke as brake shoes wear Correct answer.
  3. To automatically drain moisture from the air tanks
  4. To automatically apply the parking brake when air pressure is lost

Explanation

Automatic slack adjusters adjust the pushrod stroke to compensate for brake shoe wear, keeping the stroke within specification. They do not apply parking brakes, drain moisture, or adjust compressor pressure.

Question 36Air Brake System Diagnosis and Repair

Which component on a school bus air brake system holds the compressed air that is delivered to the brake chambers during a normal service application?

  1. The compressor discharge line
  2. The application valve exhaust port
  3. The supply reservoir
  4. The service reservoir Correct answer.

Explanation

Service reservoirs store air downstream of the supply reservoir and feed the brake valves that apply the brakes. Supply reservoirs feed auxiliary and other circuits, and the compressor discharge line is not a storage volume. The exhaust port is only a vent path.

Question 37Air Brake System Diagnosis and Repair

A school bus driver reports that the low-pressure warning light and buzzer do not activate when system pressure falls below the required threshold. Which item should the technician check first?

  1. The brake chamber pushrod length
  2. The warning switch and its electrical circuit Correct answer.
  3. The treadle valve return spring
  4. The slack adjuster free play

Explanation

The low-pressure warning is triggered by a pressure switch that completes an electrical circuit to the lamp and buzzer. If the warning does not occur at the specified pressure, the switch or its wiring is the first suspect. Mechanical brake adjustments do not control the warning circuit.

Question 38Air Brake System Diagnosis and Repair

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

  1. To supply air to the stop lamps
  2. To stop air from flowing back toward the compressor
  3. To drain moisture from the bottom of the tank
  4. To release excess pressure before it exceeds the reservoir rating Correct answer.

Explanation

The safety valve is a pressure relief device that opens if reservoir pressure rises above a set value, protecting the tank from overpressure. Drain cocks handle moisture, and check valves control backflow. Stop lamps are electrical.

Question 39Air Brake System Diagnosis and Repair

A school bus air brake system is being inspected. The technician hears a continuous hiss at the brake chamber with the brakes released. What is the most likely cause?

  1. A leaking service diaphragm or chamber seal Correct answer.
  2. A worn treadle valve roller
  3. A clogged air filter in the compressor
  4. A misadjusted governor cut-in setting

Explanation

A continuous leak at the chamber with the brakes released points to a failed diaphragm or chamber seal that cannot hold pressure. Compressor intake and governor settings affect build and cycling, not a steady chamber leak. Treadle roller wear affects pedal feel and application.

Question 40Air Brake System Diagnosis and Repair

During a compressor build-up test, a school bus takes an excessively long time to raise reservoir pressure from 85 to 100 psi. Which condition is most likely responsible?

  1. A service reservoir drain left slightly open
  2. A weak return spring in a brake chamber
  3. A slipping or glazed compressor drive belt Correct answer.
  4. An incorrectly adjusted slack adjuster

Explanation

Slow pressure buildup with the engine running usually indicates the compressor is not being driven properly or is worn. A slipping belt reduces compressor speed and output. An open drain would create a continuous leak and prevent buildup rather than merely slow it, and brake hardware does not affect compressor output.

Question 41Air Brake System Diagnosis and Repair

A technician is checking governor operation on a school bus. The compressor cuts out at 125 psi and cuts back in at 100 psi. What does this indicate?

  1. The governor cut-out is too low and should be raised
  2. The compressor is worn and should be replaced
  3. The safety valve is stuck open
  4. The governor is operating within a typical range Correct answer.

Explanation

A typical governor cut-out is around 120 to 125 psi and cut-in around 100 psi, giving a normal cycling range. These readings are consistent with correct operation. A stuck safety valve would vent continuously, and a worn compressor would slow buildup rather than change the governor set points.

Question 42Air Brake System Diagnosis and Repair

A school bus with air brakes has one rear brake chamber whose pushrod stroke exceeds the adjustment limit. What is the correct first action?

  1. Disconnect the spring brake
  2. Replace the chamber immediately
  3. Increase governor cut-out pressure
  4. Adjust the slack adjuster and recheck stroke Correct answer.

Explanation

An overstroke means the slack adjuster is out of adjustment or the foundation brake hardware is worn. The correct first step is to adjust the slack adjuster and recheck stroke against the limit. If stroke cannot be brought within limits, further diagnosis of the chamber and hardware is needed.

Question 43Air Brake System Diagnosis and Repair

A school bus air brake system uses an alcohol evaporator or air dryer to control moisture. Why is moisture control important in an air brake system?

  1. Moisture increases reservoir pressure beyond the safety valve setting
  2. Moisture can freeze in lines and valves and cause brake failure Correct answer.
  3. Moisture lubricates the compressor and improves output
  4. Moisture reduces the spring brake holding force

Explanation

Water vapor condenses in reservoirs and lines; in cold weather it can freeze and block valves or lines, causing brake failure. Moisture also promotes corrosion. It does not raise pressure or lubricate the compressor.

Question 44Air Brake System Diagnosis and Repair

A school bus has a dual-circuit air brake system. One circuit loses pressure, but the other circuit still holds pressure. What is the purpose of the pressure protection valve in this situation?

  1. To vent all reservoir pressure to atmosphere
  2. To apply the spring brakes automatically
  3. To isolate the failed circuit and preserve pressure in the healthy circuit Correct answer.
  4. To increase compressor output to the failed circuit

Explanation

Pressure protection valves close when pressure in one circuit drops, preventing the healthy circuit from being drained through the failed one. This preserves braking capability on the intact circuit. They do not boost compressor output, apply spring brakes, or intentionally vent all pressure.

Question 45Air Brake System Diagnosis and Repair

A school bus driver reports that the spring brakes will not release after the engine is started. The system pressure gauge reads 110 psi. What is the most likely cause?

  1. A misadjusted slack adjuster
  2. A clogged compressor intake filter
  3. A worn service brake diaphragm
  4. A faulty spring brake control valve or its supply line Correct answer.

Explanation

Spring brakes release only when air pressure is delivered to the spring brake chambers through the control valve. With system pressure present but no release, the control valve or its supply line is the likely fault. Service diaphragms, slack adjusters, and compressor intake do not control spring brake release.

Question 46Air Brake System Diagnosis and Repair

A technician is diagnosing a school bus that pulls to one side during braking. Which air brake system condition could cause this?

  1. A restricted air line to one brake chamber Correct answer.
  2. A governor cut-out set too high
  3. A compressor with a worn drive belt
  4. A service reservoir with a closed drain cock

Explanation

If one brake chamber receives air more slowly than the other, that side applies later or with less force, causing a pull. Governor settings, compressor drive condition, and a closed drain cock affect overall pressure supply, not side-to-side balance.

Question 47Air Brake System Diagnosis and Repair

A school bus air brake system is equipped with an automatic slack adjuster. After adjustment, the technician finds the pushrod stroke is still beyond the limit and the adjuster will not hold adjustment. What is the most likely cause?

  1. An air dryer with a saturated desiccant
  2. A worn or damaged slack adjuster or foundation brake components Correct answer.
  3. A governor cut-in set too low
  4. A low-pressure warning switch failure

Explanation

An automatic slack adjuster that will not hold adjustment after resetting usually indicates a worn adjuster or worn foundation brake parts such as the S-cam, bushings, or chamber. Governor settings, dryer condition, and warning switches do not affect stroke adjustment.

Question 48Air Brake System Diagnosis and Repair

During a school bus air brake inspection, the technician notes that the compressor runs continuously and system pressure rises above the governor cut-out setting. Which component is the most likely cause?

  1. A worn treadle valve return spring
  2. A governor that is stuck in the loaded (compressing) position Correct answer.
  3. A service reservoir with an open drain
  4. A leaking service brake diaphragm

Explanation

The governor signals the compressor to unload at cut-out. If it remains in the loaded position, the compressor keeps running and pressure can rise past the set point. An open drain or leaking diaphragm would cause pressure loss, not overpressure.

Question 49Air Brake System Diagnosis and Repair

A school bus air brake system has good pressure and the brakes release normally, but the driver reports the brakes feel slow to apply. Which condition best explains a slow application?

  1. A restricted or kinked supply line to the brake chambers Correct answer.
  2. A safety valve that opens at the correct pressure
  3. A chamber with a pushrod stroke below the limit
  4. A governor cut-out set too high

Explanation

A restricted or kinked line slows air delivery to the chambers, delaying application. Correctly functioning safety and governor components and a short stroke do not slow the application rate. A short stroke would more likely affect force and travel than timing.

Question 50Air Brake System Diagnosis and Repair

During a static leak test on a school bus air brake system, the technician fully applies the service brakes and holds them while watching the dash gauge. Pressure drops 4 psi in one minute with the engine off and the system at full pressure. Which condition does this result most directly indicate?

  1. The spring brake chambers are not holding air in the parking brake circuit
  2. The compressor is not building pressure fast enough to keep up with the leak
  3. The pressure loss is within the allowable limit for a fully applied service brake system Correct answer.
  4. The governor cut-out pressure is set too low for the system

Explanation

For a fully applied service brake system on a commercial vehicle, a pressure loss of no more than 4 psi per minute (with the engine off and a fully charged system) is the accepted allowable limit. A drop at or below that threshold indicates the service brake circuit is sealing acceptably. Larger losses would point to a leak in the applied service circuit.