Air Brakes and Braking Systems Flashcards

A technician is diagnosing an air brake system where the compressor cycles on and off too frequently. The system builds to the correct cut-out pressure, but then pressure drops rapidly to the cut-in point with no brakes applied. Which of the following is the MOST likely cause?

Answer: A leaking supply reservoir check valve

A leaking check valve at the supply (wet) tank allows compressed air to bleed back towards the compressor's unloader mechanism when the compressor cuts out. This causes a rapid drop in reservoir pressure, signaling the governor to cut the compressor back in, resulting in frequent cycling.

Which of the following BEST describes the operating principle of a spring brake chamber for parking and emergency braking?

Answer: A large coil spring provides the mechanical force to apply the brakes when air pressure is released or lost.

Spring brakes are a fail-safe system. A powerful internal spring is held in a compressed (caged) state by air pressure during normal operation. When the driver pulls the parking brake valve or in the case of air loss, the air is exhausted from the chamber, allowing the spring to expand and mechanically apply the brakes.

While performing a brake inspection, a technician measures the applied pushrod stroke of a standard, clamp-type, Type 30 brake chamber at 2.25 inches. According to CVSA (Commercial Vehicle Safety Alliance) out-of-service criteria, what is the correct action?

Answer: Place the vehicle out-of-service and adjust the slack adjuster.

The CVSA adjustment limit for a standard Type 30 brake chamber is 2.0 inches. A measurement of 2.25 inches exceeds this limit, making the brake defective. The technician must place the vehicle out-of-service and then diagnose the cause (e.g., adjust the slack adjuster, inspect foundation brake components) to bring the stroke within the legal limit.

What is the primary function of the tractor protection valve?

Answer: To prevent the tractor's air supply from being depleted by a major leak on the trailer side.

The tractor protection system is a safety feature that isolates the tractor's air brake system from the trailer's. If there is a catastrophic air loss on the trailer, such as a ruptured line or breakaway, the tractor protection valve will close, preserving air pressure for the tractor's brakes.

A driver reports that the trailer ABS warning lamp on the dashboard is continuously illuminated, but the tractor ABS lamp functions normally (cycles on and off at startup). What is the MOST likely location of the fault?

Answer: A faulty wheel speed sensor or wiring on a trailer axle.

The tractor and trailer have separate ABS systems that communicate. If the tractor's ABS light is off but the trailer's is on, the fault lies within the trailer's ABS components. A failed wheel speed sensor or damaged wiring to the sensor is the most common cause of a persistent trailer ABS fault.

During the purge cycle of an air dryer, which of the following actions occurs to regenerate the desiccant material?

Answer: A small volume of dry air from a purge tank is back-flushed through the desiccant to expel moisture.

When the air system reaches the governor cut-out pressure, the compressor unloads, and the air dryer purges. It uses a small amount of already dried air (from its own purge volume or the supply tank) and directs it backward through the desiccant bed. This rapid expansion of dry air absorbs and carries the trapped moisture out through the purge valve.