CDL Air Brakes TestAir Brake System PartsMedium
A commercial vehicle is equipped with a dual air brake system. During a pre-trip inspection, a driver observes that the primary air tank gauge reads 90 psi, while the secondary air tank gauge reads 120 psi. The air compressor governor is set to cut out at 125 psi. What is the most likely cause of this discrepancy?
- AA restriction in the air line leading to the secondary air tank.
- BA leak in the primary air tank or its associated lines/components.
- CA faulty air compressor governor, failing to cut out at the correct pressure.
- DA malfunctioning check valve between the primary and secondary air systems.
Show answer & explanationAnswer & explanation
Correct answer: B. A leak in the primary air tank or its associated lines/components.
In a dual air system, both tanks should typically be at or near the governor's cut-out pressure. If one tank's pressure is significantly lower (90 psi) while the other is near cut-out (120 psi), it indicates a loss of air from the lower-pressure tank due to a leak.
Why the other options are wrong
- A. A restriction would prevent air from reaching the tank, but a leak explains the lower pressure while the other system is full.
- C. The secondary tank reaching 120 psi suggests the governor is functioning correctly for that circuit.
- D. A malfunctioning check valve between systems usually prevents air from flowing from one system to the other, but wouldn't explain one tank being consistently lower while the other is full, unless it's preventing the primary from filling.
Dual Air Brake System
An air brake system with two independent air brake circuits (typically one for the front axle and one for the rear axle), each with its own air tank, lines, and components, designed for redundancy and safety.
- Provides a safety margin if one system fails.
- Front and rear brakes operate independently.
- Requires separate gauges for each system.
Memory trick: One Low, One High? Must Be a Leak or a Block.