Six psi in a Minute: Reading a Leak-Down on a Loaded Combination
You are doing a pre-trip on a 2016 three-axle tractor pulling a loaded two-axle 53-foot van. With the engine off, you build to governor cut-out and let the system sit for a full minute to stabilise; the gauge reads 118 psi. Wheels chocked, parking brake released, you time one minute: the gauge falls from 118 to 112 psi. You then hold the treadle fully down and time another minute: the gauge falls from 112 to 104 psi. Soapy water on the trailer's red gladhand coupler gives a slow, steady bubble that reforms no matter how tight you seat the coupler. There is also a faint hiss near the trailer's rear axle that you cannot pinpoint in the yard noise. With the dash valve pushed in, the system does recharge to cut-out, but the compressor cycles far more often than you remember. Dispatch wants you rolling in ten minutes; the nearest shop is 120 miles away and the route crosses a six-percent grade.
1. State the leakage limits that apply and decide whether this unit passes. Show the comparison.
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The leak test has two halves because a released system and an applied system pressurise different volumes.
- Brakes released: a combination vehicle should hold within about 3 psi per minute; a single vehicle within about 2 psi. You measured 6 psi. Fail.
- Brakes applied: the allowance is about 4 psi per minute for a combination, 3 psi for a single unit. You measured 8 psi. Fail.
The numbers are not marginal - they are roughly double the limit on the released side and double on the applied side, so the answer does not turn on which standard you quote. Always confirm the exact limit against the vehicle's specification, then apply it.
Because the released test already fails, nothing downstream rescues it. This is a leaking vehicle, not a gauge quirk.
2. What does the difference between the released rate and the applied rate tell you about where the air is going?
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The applied rate is naturally higher, because pressing the treadle puts the chambers, the service lines and the relay valve under pressure - more pressurised volume, more places to leak. A small increase is expected.
What matters is the size of the increase and the corroborating evidence:
- 6 psi/min released points to a leak in the supply side - something that is under pressure even when you are not braking. The bubbling red gladhand coupler fits exactly: that is the emergency/supply line, and it is charged whenever the dash valve is in.
- 8 psi/min applied adds only 2 psi, which is consistent with the same supply leak plus a modest service-side contribution. It does not suggest a large chamber or diaphragm failure.
So the primary fault is at the trailer supply coupling, with the rear-axle hiss as a possible second, smaller leak. The coupler is the cheap, obvious one to fix first - and fixing it may bring both numbers under the limit.
3. Roll or hold? Justify the decision and describe what has to happen before the truck leaves the yard.
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Hold. Three reasons, in order of severity:
- The trailer will lose supply air. The emergency line is the trailer's reservoir feed. A leaking coupler means the trailer's tanks charge more slowly than they drain, and when they fall far enough the trailer's spring brakes apply on their own - possibly at 55 mph on the six-percent grade.
- The compressor is already working overtime. A system that cycles constantly runs hot and cannot keep up with a long downgrade where every application costs air.
- The leak masks real problems. With the system bleeding 6–8 psi a minute, you cannot perform a meaningful applied-leak test or spring-brake check.
Before it moves: replace the gladhand seal (a consumable, not a repair), re-seat and re-test, chase the rear-axle hiss with soapy water, repeat both leak tests, then re-check build-up time and the low-pressure warning. If the numbers come in under the limit, it rolls. If not, it goes to the shop - the 120 miles is not the issue; the grade is.