SD CDL - South Dakota Commercial Drivers License Air Brake Systems Questions and Answers — Questions and Answers
Question 1: During a static air brake leakage test on a single vehicle, what is the maximum permissible pressure drop in one minute after the initial drop?
- 5 psi
- 2 psi (Correct answer)
- 10 psi
- 1 psi
Correct answer: 2 psi
According to FMCSA regulations for a static leakage test (engine off, brakes released), the pressure drop should not exceed 2 psi per minute for a straight truck or bus. For combination vehicles, the limit is 3 psi per minute.
Question 2: A driver is operating a truck with a dual air brake system. The low air pressure warning light and buzzer activate. What is the most likely pressure range in one of the systems?
- 80-90 psi
- 100-120 psi
- Below 60 psi (Correct answer)
- 20-45 psi
Correct answer: Below 60 psi
The low air pressure warning devices (light, buzzer, or wig wag) are required to activate before the air pressure in either the primary or secondary system drops below 60 psi.
Question 3: Which of the following components is responsible for stopping the air compressor from pumping air when the storage tanks reach their maximum pressure?
- The safety valve
- The brake pedal
- The air compressor governor (Correct answer)
- The slack adjuster
Correct answer: The air compressor governor
The air compressor governor controls when the air compressor pumps air. It stops the compressor (cuts out) when the tank pressure reaches the specified maximum (typically 125-140 psi) and allows it to start pumping again (cuts in) when the pressure drops to around 100 psi.
Question 4: A driver is preparing for their pre-trip inspection. If the vehicle is equipped with an alcohol evaporator, what is its primary function?
- To clean the air going into the tanks
- To reduce the risk of ice forming in air brake valves during cold weather (Correct answer)
- To inject alcohol into the fuel system for better combustion
- To cool the air compressed by the engine
Correct answer: To reduce the risk of ice forming in air brake valves during cold weather
The alcohol evaporator puts alcohol vapor into the air system. This helps to absorb moisture and reduce the chance that water will freeze in the air brake valves or lines, which could cause brake failure in cold temperatures.
Question 5: Under normal driving conditions, what holds the spring brakes in the released position?
- Hydraulic pressure
- Mechanical linkage from the brake pedal
- The driver manually holding a lever
- Air pressure (Correct answer)
Correct answer: Air pressure
Spring brakes are a safety feature. They apply automatically when air pressure drops to a low level (20-45 psi). During normal operation, sufficient air pressure in the system overcomes the force of the powerful springs, keeping them caged or released.
Question 6: When descending a long, steep grade, a driver should brake until the vehicle's speed is about 5 mph below the 'safe speed' and then release the brakes. Why is this 'snub braking' technique recommended?
- To allow the brake drums to cool and prevent brake fade (Correct answer)
- To keep the air pressure from dropping too quickly
- To ensure the trailer brakes are working harder than the tractor brakes
- To dry out wet brake linings
Correct answer: To allow the brake drums to cool and prevent brake fade
Constant, light pressure on the brakes can cause them to overheat, leading to a dangerous condition called brake fade where they become less effective. The snub braking technique of applying firm pressure for a short time and then releasing allows the brakes to cool between applications, maintaining their stopping power.
SD CDL - South Dakota Commercial Drivers License Air Brake Systems Questions and Answers
During a static air brake leakage test on a single vehicle, what is the maximum permissible pressure drop in one minute after the initial drop?