Air Brake Systems Flashcards

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?

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.

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?

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.

Which of the following components is responsible for stopping the air compressor from pumping air when the storage tanks reach their maximum pressure?

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.

A driver is preparing for their pre-trip inspection. If the vehicle is equipped with an alcohol evaporator, what is its primary function?

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.

Under normal driving conditions, what holds the spring brakes in the released position?

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.

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?

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.