A technician is diagnosing a commercial vehicle's air brake system. They observe that when the service brakes are applied, the air pressure in both the primary and secondary systems drops simultaneously and significantly, even though the compressor is running. What is the most likely cause?
- AA faulty air dryer allowing moisture into the system.
- BA clogged air filter preventing adequate air intake.
- CExcessive air consumption by the service brakes due to maladjustment or leaks.
- DA stuck open safety relief valve on the air tanks.
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive air consumption by the service brakes due to maladjustment or leaks.
If both primary and secondary system pressures drop significantly and simultaneously upon brake application, and the compressor cannot keep up, it indicates excessive air consumption. This is typically caused by maladjusted brakes (requiring more air volume to actuate) or multiple leaks in the brake chambers or lines that only become apparent when air is applied to them.
Why the other options are wrong
- A. A faulty air dryer primarily leads to water contamination, not direct excessive air loss during braking.
- B. A clogged air filter would reduce the compressor's output, leading to slow air build-up, but not necessarily a rapid, simultaneous drop upon brake application if the compressor is running.
- D. A stuck open safety relief valve would cause a constant, rapid air loss from the tank it's on, regardless of brake application, and would likely be very noisy.
Excessive Air Consumption
When a dual air brake system loses pressure rapidly in both primary and secondary systems upon brake application, it often indicates excessive air consumption due to maladjusted brakes or leaks in the brake components.
- Both gauges drop significantly.
- Occurs during brake application.
- Compressor struggles to maintain pressure.
- Common causes: maladjusted brakes, leaks in chambers/lines.
Memory trick: When both gauges go down, the brakes demand too much air around.