CDL Air Brakes TestAir Brake System PartsHard
A driver is conducting a pre-trip inspection on a commercial vehicle with a dual air brake system. They notice that the primary air tank gauge reads 125 psi, but the secondary air tank gauge reads 0 psi. There is no audible air leak. What is the most likely cause of this discrepancy?
- AThe parking brake is engaged, causing the secondary tank to drain.
- BA faulty one-way check valve between the tanks or a blocked line to the secondary tank.
- CA malfunctioning air compressor.
- DA major leak in the primary air tank.
Show answer & explanationAnswer & explanation
Correct answer: B. A faulty one-way check valve between the tanks or a blocked line to the secondary tank.
In a dual air system, check valves prevent air from flowing out of one tank if the other fails. If the primary tank is full but the secondary is empty with no leak, it suggests air isn't reaching the secondary tank, likely due to a stuck or failed check valve preventing air flow into it, or a blockage in the line.
Why the other options are wrong
- A. The parking brake uses air from the secondary (or primary, depending on design) but shouldn't cause it to read 0 psi if the system is otherwise pressurized and no leak is present.
- C. A malfunctioning compressor would prevent both tanks from filling, or at least one, but here the primary is full.
- D. A major leak in the primary tank would cause its pressure to drop, not remain full.
Dual Air System Check Valve
One-way valves in a dual air brake system that separate the primary and secondary air circuits, ensuring that a leak in one system does not completely drain the other.
- Prevents total air loss from a single fault.
- Allows air to flow into tanks, but not back out.
- Failure can lead to one tank not pressurizing.
Memory trick: Check Valves Control Flow, Keeping Tanks Apart.