CDL Air Brakes Practice Test 4 — Questions and Answers
Question 1: Which device allows you to remove oil and water from the air brake systems?
- Air tank drain (Correct answer)
- Safety valve
- Alcohol evaporator
- Treadle valve
Correct answer: Air tank drain
Compressed air naturally contains moisture, and over time, oil from the compressor can also enter the system. These contaminants collect in the bottom of the air tanks. Air tank drains, located at the bottom of each tank, allow drivers to manually or automatically remove this accumulated water and oil, preventing corrosion, freezing, and damage to other air brake components.
Question 2: What does it mean when the safety valve opens and releases air?
- The pressure has exceeded 150 psi
- The system is fully charged
- Something is wrong, and a mechanic needs to check it
- A and C (Correct answer)
Correct answer: A and C
The safety valve is designed to protect the air brake system from excessive pressure, typically opening and releasing air if the pressure in the tanks exceeds 150 psi. If this valve frequently opens, it indicates a malfunction in the air compressor governor, which should be controlling the pressure within the normal range. Therefore, both excessive pressure and a system malfunction are indicated, requiring immediate inspection by a mechanic.
Question 3: What should the air pressure loss be in a single vehicle with a fully charged air system?
- Less than 2 psi per minute (Correct answer)
- Less than 3 psi per minute
- Less than 4 psi per minute
- Less than 5 psi per minute
Correct answer: Less than 2 psi per minute
During a static air brake test, a single vehicle with a fully charged air system should not lose more than 2 psi in one minute after the initial pressure drop from applying the brakes. This low rate of pressure loss indicates that the air brake system is relatively free of leaks. Exceeding this limit suggests a problem that needs to be addressed for safe vehicle operation.
Question 4: In a combination vehicle, what should the air pressure loss be with a fully charged air system?
- Less than 2 psi per minute
- Less than 3 psi per minute (Correct answer)
- Less than 4 psi per minute
- Less than 5 psi per minute
Correct answer: Less than 3 psi per minute
For a combination vehicle (tractor-trailer), the acceptable air pressure loss during a static air brake test is slightly higher than for a single vehicle due to the increased complexity and number of air lines. With a fully charged system and the brakes applied, the pressure loss should not exceed 3 psi per minute. A greater loss indicates potential leaks in the system, which must be repaired for safe operation.
Question 5: Where is compressed air stored?
- Air compressor
- Pressure gauge
- Main reservoirs (Correct answer)
- Service and emergency air lines
Correct answer: Main reservoirs
Compressed air, generated by the air compressor, is stored in large metal tanks called main reservoirs or air tanks. These tanks hold a reserve of compressed air, which is then supplied to the brake chambers and other air-operated accessories. Having these reservoirs ensures a constant supply of air pressure for braking, even if the compressor is not actively pumping.
Question 6: Which of the following devices indicates how much air pressure you are applying to your brakes?
- Application pressure gauge (Correct answer)
- Low air pressure warning signal
- Governor
- Supply pressure gauge
Correct answer: Application pressure gauge
The application pressure gauge indicates how much air pressure is being sent to the brake chambers when the driver presses the brake pedal. This gauge helps the driver monitor their braking effort and ensures that the brakes are receiving adequate pressure for effective stopping. It is distinct from the supply pressure gauge, which shows the total air pressure available in the tanks.
Question 7: Which of the following devices can be used as the low air pressure warning signal?
- A buzzer
- Red warning light
- A mechanical arm (wig wag)
- Any of the above (Correct answer)
Correct answer: Any of the above
Federal regulations require commercial vehicles with air brakes to have a low air pressure warning signal to alert the driver when system pressure drops below 60 psi. This critical warning can be provided by various means, including an audible buzzer, a visual red warning light, or a mechanical arm known as a 'wig-wag.' All these devices serve the same essential safety function.
Question 8: What is a wig-wag?
- A device that cuts the normal air pressure to the front brakes by half
- A mechanical arm that comes into view when the air pressure falls below 60 psi (Correct answer)
- Another name for a brake pedal
- A lever used to apply the spring brakes gradually
Correct answer: A mechanical arm that comes into view when the air pressure falls below 60 psi
A 'wig-wag' is a specific type of low air pressure warning device found on some older commercial vehicles. It is a mechanical arm that physically drops or 'wags' into the driver's view, typically in the dashboard, when the air pressure in the brake system falls below approximately 60 psi. This visual indicator serves as a critical alert to the driver about dangerously low air pressure.
Question 9: What is a modulating control valve?
- A device that controls when the air compressor will pump air into the air storage tanks.
- A device that cuts the normal air pressure to the front brakes by half.
- A valve used to drain oil and water from the air storage tanks.
- A lever used to apply the spring brakes gradually (Correct answer)
Correct answer: A lever used to apply the spring brakes gradually
A modulating control valve is a specialized valve used in some air brake systems, particularly for vehicles with spring parking brakes. It allows the driver to gradually apply the spring brakes, rather than an all-or-nothing application. This can be useful in situations like a loss of air pressure, allowing for controlled braking rather than an abrupt stop, or for testing the spring brakes.
Question 10: What does the Air Compressor Governor control?
- When the air is released from the tank
- When air pressure is cut to the front brakes
- When the air compressor pumps air into the storage tanks (Correct answer)
- When water and oil are automatically expelled from the storage tanks
Correct answer: When the air compressor pumps air into the storage tanks
The air compressor governor is a critical component that regulates the air pressure in the air brake system. It controls the air compressor by telling it when to start pumping air into the storage tanks (cut-in pressure) and when to stop (cut-out pressure). This ensures that the system maintains optimal operating pressure without over-pressurizing the tanks.
CDL Air Brakes Practice Test 4
Which device allows you to remove oil and water from the air brake systems?