CDL Air Brakes TestDual Air Brake SystemsMedium
A commercial vehicle is equipped with a dual air brake system. The primary system supplies air to the rear service brakes, and the secondary system supplies air to the front service brakes. During a pre-trip inspection, the driver notes that the air pressure gauge for the primary system reads 90 psi, while the secondary system gauge reads 120 psi. What is the most likely cause for this pressure discrepancy?
- AThe secondary system is over-pressurized.
- BA faulty air compressor governor.
- CA leak in the primary air system.
- DThe parking brake is applied, drawing air from the primary system.
Show answer & explanationAnswer & explanation
Correct answer: C. A leak in the primary air system.
A significant pressure difference between the primary and secondary systems, with one being lower, typically indicates a leak in the lower-pressure system. If the compressor is working (as evidenced by the 120 psi in the secondary system), it's likely trying to keep both full, but the primary system is losing air.
Why the other options are wrong
- A. 120 psi is within the normal operating range (100-125 psi); it's not over-pressurized.
- B. A faulty governor would likely affect both systems or prevent any system from reaching full pressure, not just one.
- D. The parking brake typically drains air from the primary system (or sometimes a separate tank), but it would cause a drop, not a sustained 90 psi while the secondary is at 120 psi if the compressor is running. A leak is more likely for a sustained difference.
Dual System Pressure Discrepancy
A noticeable and sustained difference in air pressure between the primary and secondary air systems often indicates a leak in the lower-pressure system.
- Compressor tries to fill both
- Lower pressure system has issue
- Requires investigation for leaks
Memory trick: If one side's low and the other's high, it's like a leaky bucket in the low side's sky.