CDL Air Brakes TestAir Brake System PartsMedium
A driver is inspecting a commercial vehicle equipped with a dual air brake system. They notice that the air pressure gauge for the primary air tank reads 125 psi, but the secondary air tank gauge reads only 70 psi. What is the most probable cause of this discrepancy?
- AThe secondary air tank has a major leak.
- BThe check valve between the primary and secondary tanks is faulty.
- CThe primary air tank's safety valve has opened prematurely.
- DThe air compressor is only supplying air to the primary tank.
Show answer & explanationAnswer & explanation
Correct answer: B. The check valve between the primary and secondary tanks is faulty.
In a dual air brake system, a check valve ensures that air flows from the primary tank to the secondary tank, but not back. If the secondary tank pressure is significantly lower despite the primary tank being full, it indicates that the check valve is preventing proper air transfer to the secondary tank.
Why the other options are wrong
- A. While a major leak in the secondary tank would cause low pressure, the question implies the primary tank is full, suggesting the issue is with air getting to the secondary tank, not just leaking out.
- C. A premature opening of the primary tank's safety valve would cause a rapid loss of pressure in the primary tank, not a discrepancy between the tanks with the primary still at 125 psi.
- D. The air compressor supplies air to both tanks, usually routed through a common line to the primary tank first, then via a check valve to the secondary.
Dual Air System Check Valve
A check valve in a dual air brake system allows air to flow from the primary air tank to the secondary air tank, but prevents air from flowing back, ensuring that a loss of pressure in one system does not completely drain the other.
- Maintains separation of primary and secondary air supplies.
- Critical for system redundancy and safety.
- Failure can lead to imbalanced pressures or loss of one system.
Memory trick: Two tanks, one flow. If the check valve's stuck, the pressure won't go!