System Components & Functions Flashcards
6 cards from real Air Brake Test practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 System Components & Functions flashcards as text
What is the role of the limiting valve (formerly called a front brake limiting valve) in an air brake system?
Answer: It reduces air pressure delivered to the front axle brakes during light load conditions to prevent front wheel lockup on slippery roads
Older limiting valves reduce front axle brake pressure in slippery conditions to prevent front wheel lockup, though many modern vehicles with ABS no longer require manual limiting valves.
What does the tractor protection valve do, and when does it activate?
Answer: It closes to isolate the tractor air supply from the trailer when trailer air pressure drops too low, preventing the tractor from losing all its air
The tractor protection valve senses trailer air pressure and closes at approximately 20–45 psi, sealing tractor air from the trailer to preserve tractor braking ability if the trailer has a major air loss.
What is the purpose of the emergency relay valve on a trailer?
Answer: To apply the trailer spring brakes if the supply air line pressure drops, serving as the trailer's emergency braking system
The emergency relay valve monitors supply line pressure; if it drops (indicating trailer separation or supply failure), it applies the trailer spring brakes automatically as an emergency measure.
What is the function of the anti-compounding valve in a spring brake system?
Answer: It prevents simultaneous application of both service brakes and spring brakes, which could over-stress brake components
The anti-compounding valve prevents both service and spring brakes from applying together, as the combined force could exceed the structural limits of brake chamber and foundation brake components.
What is the primary purpose of the two air tanks (primary and secondary) in a dual air brake system?
Answer: To provide separate, independent air supplies for front and rear brakes so a single circuit failure does not result in complete brake loss
Having separate primary and secondary tanks means that a failure in one circuit (such as a broken line) does not drain the other circuit, preserving partial braking capability on the remaining axles.
What component in an air brake system converts air pressure into mechanical force at each wheel?
Answer: The brake chamber (air chamber)
The brake chamber contains a flexible diaphragm that air pressure pushes against, moving the pushrod outward and converting pneumatic pressure into the mechanical force that applies the brakes.