Single-Vehicle Leak Test: Static Pass, Applied Fail
You are doing a pre-trip on a 2017 Freightliner M2 26,000-lb box truck with air brakes. You build pressure to 120 psi, shut off the engine, chock the wheels, and release the parking brake. With the service brake released, you note the gauge and wait one minute: it drops from 120 to 118 psi. You then press the service brake pedal fully and hold it for one minute: the gauge drops from 118 to 113 psi. The truck is a single vehicle, not a combination. Your dispatcher says the truck is needed for a 40-mile local run and asks if it is "close enough." The parking brake holds the truck on a slight incline. There is no audible leak at idle, but you hear a faint hiss near the left rear wheel when the brake is applied.
1. Evaluate both leak tests against the correct limits for this vehicle.
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This is a single vehicle, so use the single-vehicle limits, not tractor-trailer limits.
- Static test (brake released): 120 to 118 psi = 2 psi loss. Limit is 3 psi for a single vehicle. Pass.
- Applied test (brake held): 118 to 113 psi = 5 psi loss. Limit is 4 psi for a single vehicle. Fail.
The dispatcher's "close enough" reasoning is wrong. The applied test is the one that matters when you are actually trying to stop, and 5 psi exceeds the allowable loss. The faint hiss at the left rear wheel under application supports a real service-side leak, not gauge error.
2. What does the applied-test failure indicate, and which components would you inspect?
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A leak that appears only when the brake pedal is applied is in the service brake circuit. Likely locations include:
- the left rear brake chamber diaphragm;
- the hose or fitting at that chamber;
- a relay valve or quick-release valve;
- the treadle valve under the pedal; or
- a cracked line that opens under pressure.
The hiss at the left rear wheel narrows the first check to that chamber, hose, and fitting. Do not assume the chamber is the only problem; air can travel through the system and fool you about the source. Use soapy water and listen carefully with the brake applied.
3. How would you answer the dispatcher, and would the answer change if this were a tractor-trailer?
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Answer: No-go. A 5 psi applied leak exceeds the 4 psi single-vehicle limit and needs repair before dispatch. The parking brake holding on an incline does not prove the service brakes are sound; the spring brakes are a separate system.
If this were a tractor-trailer, the limits would be different: typically 4 psi static and 6 psi applied for a combination. A 5 psi applied loss would be within the combination applied limit, but it would still be a leak that should be identified and monitored. For this single truck, it is over the limit, so the distinction matters. Do not apply combination limits to a straight truck.