CDL Air Brakes TestDual Air Brake SystemsHard
During a pre-trip inspection, a driver observes that the air pressure in the primary system builds to 125 psi, but the secondary system only builds to 100 psi before the governor cuts out the compressor. The check valve between the tanks appears to be functioning. What is the most likely issue?
- AA restriction in the air line leading to the primary system.
- BA leak in the secondary air system.
- CAn incorrectly set governor cut-out pressure.
- DA faulty air pressure gauge for the secondary system.
Show answer & explanationAnswer & explanation
Correct answer: B. A leak in the secondary air system.
If the primary system reaches full pressure and the governor cuts out, but the secondary system is significantly lower (100 psi vs. 125 psi), and the check valve is good (meaning air isn't flowing back), the most likely cause is a leak in the secondary system that prevents it from reaching full pressure before the compressor stops, or a restriction in the line feeding the secondary tank.
Why the other options are wrong
- A. A restriction in the primary system's line would prevent the primary from reaching 125 psi, which it did.
- C. The governor cut-out pressure is typically set for the *entire* system, often by sensing the primary tank. If it cuts out at 125 psi, that's normal. The issue is why the secondary didn't reach that pressure.
- D. While possible, a gauge fault wouldn't explain why the system *only* builds to 100 psi and then stops. The system itself would still try to build to 125 psi.
Uneven Pressure Build-up
If one air system reaches full pressure and the compressor cuts out, but the other system is significantly lower, it indicates a leak or restriction in the lower-pressure system.
- Compressor stops at cut-out
- Lower system has an issue
- Requires leak/restriction search
Memory trick: If one tank is full and the other's not, the low one has a 'leak' or 'knot'.