Air Leak on the Pre-Trip at 6:10 a.m.
At 06:10 at a truck stop north of Baton Rouge you are doing the pre-trip on a combination vehicle: a three-axle tractor pulling a two-axle semitrailer. Engine at operating temperature, 78°F outside, no load in the trailer. You build system pressure to 120 psi, shut the engine off, release the parking brake control, and let the system sit. The gauge is steady through the first minute. You then time it: it reads 118 psi, and exactly one minute later it reads 112 psi. With the engine running you bleed pressure down to test the warning devices. The low-air buzzer sounds at 58 psi and the tractor protection valve pops at 40 psi. There are no audible leaks at the gladhands or under the hood. The compressor cuts back in at 100 psi and out at 125 psi. Your dispatcher has called twice asking when you will roll - the load is due in Houston at 14:00.
1. Does this vehicle pass the static air leak test? Show the arithmetic and state the standard you are applying.
Read the worked answer
It fails. With the engine off and the parking brake control released, a combination vehicle may lose no more than 4 psi per minute; a single-unit vehicle gets 3 psi/min. The minute right after you release the control includes some normal settling, and 120 → 118 psi is only 2 psi, so that reading is not the one to judge by. The stabilized minute is the one that counts: 118 → 112 psi is 6 psi in one minute, which is 50 percent over the tractor-trailer limit.
Two of the three checks pass and one does not:
- Leak rate: fail (6 psi/min vs. 4 psi/min allowed).
- Low-air warning: fail - the warning must come on before pressure drops below 60 psi. It sounded at 58 psi, meaning the gauge was already below 60 when the driver was first warned.
- Tractor protection valve: pass - it should close in roughly the 20–45 psi range, and 40 psi is inside that window.
The compressor cut-in/cut-out of 100/125 psi is normal and tells you nothing about the leak.
2. The dispatcher wants you to run the load and let a shop look at it in Houston. Explain the mechanical risk of that decision and what you do next.
Read the worked answer
A 6 psi/min leak is small at the gauge and large in service. A compressor can usually keep up at highway rpm, which is what makes this defect feel harmless on the first 100 miles - but in stop-and-go traffic, at a dock, or at idle the compressor cannot keep pace with the loss, and pressure walks downward every time you cycle the brakes.
The failure mode is not gradual: when system pressure falls into roughly the 20–45 psi range, the spring brakes apply automatically and the vehicle stops where it is - potentially in an intersection or on a downgrade. A brake system that cannot hold pressure is an out-of-service condition, not a maintenance item for later.
So the answer to the dispatcher is no, in writing, on the DVIR. The driver holds the CDL; "the company told me to" is not a defense for moving a vehicle with a known brake defect.
3. Two defects are recorded. Which do you have repaired first, and why does the order matter?
Read the worked answer
Repair the leak first. The warning device is a messenger, not the disease: a warning buzzer that comes on at 58 psi instead of above 60 psi costs you reaction margin, but it does not by itself cause a loss of braking. The leak does. Fixing the switch on a system that still bleeds 6 psi/min leaves you with a truck that loses air faster than the compressor replaces it and a warning that now tells you about it slightly earlier.
Order matters because you verify the second defect with a healthy system. Once the leak is sealed, rebuild to 120 psi and re-run both tests: the stabilized loss must be at or under 4 psi/min, and the low-air warning must activate while the gauge is still above 60 psi. Only when both pass does the vehicle leave the lot.