Pre-Trip Leak Test Shows a Slow Drop
Before a morning run, you build air pressure to 120 psi, shut off the engine, and let the system settle. With the brakes released, the gauge drops from 120 to 112 psi over one minute. You then apply and hold the brakes; the gauge drops from 112 to 98 psi over the next minute. The low-air warning does not sound during either check. It is 28 degrees F and the truck sat outside overnight. The shop is 40 minutes away and your first delivery is 90 minutes out. Your co-driver says the drop is normal for cold weather and wants to leave.
1. Evaluate these leak-test results against typical acceptance criteria and decide whether to depart.
Read the worked answer
Typical acceptance criteria for a tractor-trailer are a pressure loss of no more than about 3 psi per minute with the brakes released and no more than about 4 psi per minute with the brakes applied (the exact limits vary by vehicle and jurisdiction, so confirm the spec for your equipment). Your results are far outside those limits: about 8 psi per minute released and about 14 psi per minute applied. That is a real leak, not normal cold-weather behavior. Cold can affect gauge response and condensation, but it does not explain a drop of that size.
Decision: do not depart. The vehicle is not roadworthy until the leak is found and corrected. A leak that size will worsen under vibration and heat, and it erodes your safety margin on every stop. The correct action is to notify dispatch, document the readings, and get the system repaired or the vehicle swapped.
2. Where would you look for the leak, and how would you isolate it?
Read the worked answer
Work from the supply side outward:
- Check the compressor, governor, and unloader for continuous cycling or a stuck unloader.
- Inspect supply and service lines, gladhands, and trailer connections for cuts, loose fittings, or missing seals.
- Listen along the frame rails, at relay valves, quick-release valves, and brake chambers.
- Check drain cocks on air tanks; a partially open drain will bleed pressure steadily.
- Look for oil or water mist at fittings, which indicates a leak.
To isolate, charge the system, shut off the engine, and watch the gauges. If the drop is on one circuit only, focus there. Disconnect the trailer and retest the tractor alone; if the leak persists, it is on the tractor. Apply soapy water to suspect fittings and look for bubbles. If the leak stops when the brakes are released, suspect the service side; if it continues with brakes released, suspect supply or a drain. Do not road-test to find a leak.
3. What would you tell the co-driver, and what is the operational consequence?
Read the worked answer
Explain that cold weather does not justify an 8 psi per minute drop with brakes released and 14 psi per minute applied. Those numbers exceed typical limits by a wide margin and indicate an active leak that will only get worse. A driver who departs with a known leak is accepting a preventable risk and may be cited or placed out of service at an inspection.
Operational consequence: the run is delayed. Notify dispatch, request a repair or a substitute tractor, and document the gauge readings and time. If the shop is 40 minutes away, arrange to have the vehicle inspected there rather than driving it loaded on a schedule. The delivery may need to be re-routed or reassigned. No schedule pressure justifies departing with a failed leak test.