Free Air Brake MCQ Question and Answer — Questions and Answers
Question 1: How much force is applied to the push rod in a type 30 brake chamber when a 40 PSI brake application is made?
- 70 PSI
- 400 PSI
- 1200 PSI (Correct answer)
- There is no such thing as a type 30 brake chamber
Correct answer: 1200 PSI
The force applied to a brake chamber's push rod is calculated by multiplying the chamber's effective diaphragm area by the air pressure. A Type 30 brake chamber has an effective area of 30 square inches. Therefore, a 40 PSI brake application generates 30 sq. in. * 40 PSI = 1200 pounds of force.
Question 2: When the air system reaches operational pressure, what does the compressor do?
- It has an internal cooling system
- Goes into an unloaded stage (Correct answer)
- Temporarily stops running
- Nothing. The compressor is supposed to run hot
Correct answer: Goes into an unloaded stage
When the air system reaches its maximum operational pressure, the governor signals the compressor to enter an 'unloaded' stage. In this stage, the compressor continues to run but stops pumping air into the reservoirs. This prevents over-pressurization of the system and reduces wear on the compressor, conserving energy until the pressure drops again.
Question 3: Between what two pressures should reservoir pressure be maintained?
- 100 (689 kPa) psi to 125 psi.
- 50 psi (345 kPa) to 90 psi (620 kPa).
- 80 (552 kPa) psi to 135 psi (930 kPa). (Correct answer)
- 105 (724 kPa) psi to 150 psi (1034 kPa).
Correct answer: 80 (552 kPa) psi to 135 psi (930 kPa).
For most heavy-duty vehicles equipped with air brakes, the air pressure in the reservoirs should be maintained within a specific operational range to ensure sufficient air supply for safe braking. This range typically falls between 80 psi (552 kPa) and 135 psi (930 kPa), which is crucial for vehicle safety and regulatory compliance.
Question 4: When conducting a build-up test on the compressor, what pressure do you start at?
- 50 psi (345 kPa). (Correct answer)
- 100 psi (690 kPa).
- 85 psi (587 kPa).
- When the low-air warning activates.
Correct answer: 50 psi (345 kPa).
When conducting an air compressor build-up test, the procedure typically begins with the air pressure in the system at a specific low point. Starting at 50 psi (345 kPa) allows for an accurate measurement of how quickly the compressor can build pressure to the governor cut-out point, ensuring it meets minimum performance standards and verifies its efficiency.
Question 5: What is the lowest pressure at which the governor should trigger the compressor to resume its pumping phase?
- 105 psi (724 kPa).
- 60 psi (414 kPa).
- 80 psi (552 kPa). (Correct answer)
- 65 psi (448 kPa).
Correct answer: 80 psi (552 kPa).
The air brake system's governor is designed to control the compressor's operation, ensuring adequate air pressure. It should trigger the compressor to resume pumping air into the reservoirs when the pressure drops to approximately 80 psi (552 kPa). This 'cut-in' pressure ensures that there is always sufficient air for safe braking, preventing the pressure from falling too low.
Question 6: What is a parking brake with a DD3 safety actuator?
- A spring brake lock.
- An air-operated parking brake.
- An automatic park brake.
- A caging device. (Correct answer)
Correct answer: A caging device.
A DD3 safety actuator is a type of spring brake chamber that incorporates a caging device. This device is used to manually compress the powerful spring inside the brake chamber, allowing the parking brake to be released for maintenance or towing when there is no air pressure available. It's a crucial safety feature to prevent the vehicle from being immobilized.
Question 7: Which three fundamental brake types are there?
- S cam brakes, disc brakes and wedge brakes. (Correct answer)
- Drum brakes, disc brakes and wedge brakes.
- S cam brakes, disc brakes and friction brakes.
- Rotary brakes, disc brakes and wedge brakes.
Correct answer: S cam brakes, disc brakes and wedge brakes.
The three fundamental types of air brakes commonly found on heavy-duty vehicles are S-cam brakes, disc brakes, and wedge brakes. S-cam brakes are the most prevalent drum brake design, while disc brakes offer improved stopping power and heat dissipation. Wedge brakes, though less common today, use a wedge mechanism to force brake shoes against the drum.
Question 8: Air loss shouldn't be more than when performing an air leak test in a single unit at maximum air pressure.
- 4 psi (28 kPa).
- 6 psi (41 kPa).
- 3 psi (21 kPa). (Correct answer)
- 2 psi (14 kPa).
Correct answer: 3 psi (21 kPa).
During an air leak test on a single unit (truck or bus) at maximum air pressure, the air loss should not exceed 3 psi (21 kPa) per minute. This standard is crucial for safety, as excessive air loss indicates leaks in the system that could compromise braking performance. Maintaining a tight air system is essential for reliable and safe operation.
Question 9: What type of pressure holds the parking or emergency brakes on trucks and buses in place?
- Fluid pressure
- Atmospheric pressure
- Spring pressure (Correct answer)
- Air pressure
Correct answer: Spring pressure
In air brake systems, the parking or emergency brakes are held in place by powerful springs, not air pressure. Air pressure is actually used to compress these springs, releasing the brakes. When air pressure is lost or released by the driver, the springs expand, applying the brakes automatically as a fail-safe mechanism.
Question 10: What kind of gradient should you park on in order to check the manual slack adjusters of the S-CAM brakes for free play?
- Park on level ground, chock the wheels and release the parking brakes (Correct answer)
- Level ground and apply the parking brake
- Park on a slight grade, release the parking brakes and apply the service brake watching your vehicle movement
- Park on level ground, drain off the air pressure before checking the adjustment
Correct answer: Park on level ground, chock the wheels and release the parking brakes
To accurately check the free play of manual slack adjusters on S-CAM brakes, it's essential to park the vehicle on level ground and chock the wheels to prevent movement. Crucially, the parking brakes must be released to ensure the brake shoes are off the drums, allowing for a proper assessment of the slack adjuster's travel without spring pressure interference.
Free Air Brake MCQ Question and Answer
How much force is applied to the push rod in a type 30 brake chamber when a 40 PSI brake application is made?