Free Air Brake MCQ Questions and Answers — 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 by a brake chamber is calculated by multiplying the air pressure by the effective area of the diaphragm. A Type 30 brake chamber has an effective area of 30 square inches. Therefore, a 40 PSI brake application on a Type 30 chamber results in 40 PSI * 30 sq. inches = 1200 pounds of force on the push rod.
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 (cut-out pressure), the air compressor governor signals the compressor to enter an 'unloaded' stage. In this stage, the compressor continues to run but stops pumping air into the reservoirs, instead diverting it back into the atmosphere or its intake. This prevents over-pressurization and reduces wear on the compressor.
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).
The air brake system's reservoir pressure should be maintained within a specific range to ensure proper braking function. The governor typically cuts in around 80 psi to start the compressor and cuts out around 135 psi to stop it. This range ensures there is always sufficient air pressure for multiple brake applications and safe operation.
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 a compressor build-up test, you typically start with the air pressure at 50 psi. From this point, you time how long it takes for the compressor to build the pressure up to the governor's cut-out pressure, usually around 125 psi. This test verifies the compressor's efficiency and proper operation.
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 compressor governor is designed to maintain air pressure within a safe operating range. It should trigger the compressor to 'cut in' and start pumping air when the reservoir pressure drops to approximately 80 psi. This ensures that there is always enough air pressure for safe braking and prevents the low-air warning from activating unnecessarily.
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 bolt or device. This caging device allows a technician to manually compress the powerful spring brake, releasing the parking brake without air pressure. This is essential for towing or moving a vehicle with no air pressure, as it 'cages' the spring.
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 commercial vehicles are S-cam drum brakes, disc brakes, and wedge brakes. S-cam brakes are the most common, using an S-shaped cam to push brake shoes against a drum. Disc brakes use calipers and pads to squeeze a rotor, while wedge brakes use a wedge to force shoes apart against a 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 (straight truck or bus) with the engine off, parking brake released, and service brake applied, the air loss rate should not exceed 3 psi in one minute. A higher loss rate indicates a significant leak that needs to be identified and repaired for 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
Parking and emergency brakes on trucks and buses are held in place by powerful springs, not air pressure. Air pressure is used to compress these springs and release the brakes. If air pressure is lost, the springs expand, automatically applying the brakes 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 manual slack adjusters for free play, the vehicle must be on level ground, and the wheels must be chocked to prevent movement. Crucially, the parking brakes must be released to ensure the service brakes are free to move, allowing for a proper measurement of slack adjuster travel.
Free Air Brake MCQ Questions and Answers
How much force is applied to the push rod in a type 30 brake chamber when a 40 PSI brake application is made?