Leakage Test Where Only the Applied Side Fails
A 2020 tractor pulling a loaded 53-foot van. Governor cut-out is 122 psi. During the morning pre-trip, engine off and at full pressure: with the brakes fully released, pressure falls from 120 to 114 psi in 2 minutes. With the treadle valve held fully applied, pressure falls from 118 to 108 psi in 1 minute, and there is a steady hiss at the left rear trailer axle that stops the instant the treadle is released. With the engine off and the driver fanning the brakes down, the low-pressure buzzer and light come on at 58 psi, and the trailer spring brakes set at 42 psi. Company tolerance for a combination is 3 psi/min released and 3 psi/min applied. The driver wants to leave: 'The warning works and the springs set, so the safety stuff is all good.'
1. Convert both leakage readings to psi per minute and rule on each against the company tolerance. Then say what the difference between the two readings tells you.
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Released: 6 psi over 2 minutes = 3 psi/min. That is exactly at the 3 psi/min tolerance - technically a pass, but with zero margin. A healthy combination typically loses well under that.
Applied: 10 psi over 1 minute = 10 psi/min. That is more than three times the tolerance. Clear failure.
The gap between the two numbers is the useful part. Released leakage exercises the system up to the treadle valve and the lines feeding it. Applied leakage adds the treadle valve's delivery circuit, the relay valves, the service hoses and - critically - the service side of every brake chamber. A leak that appears only when the treadle is down and is localised by ear at one trailer axle is a service-side leak at that wheel end, most likely a chamber diaphragm or a chafed service hose.
So the released reading tells you the system as a whole is marginal; the applied reading tells you where and how bad.
2. The driver argues that the low-pressure warning and the spring brake application pressures are both correct, so the truck is safe. Address that reasoning and explain why an applied-side leak is more dangerous than a released-side leak of the same size.
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Both of those readings are indeed normal - warning at 58 psi is ahead of the 60 psi mark, and springs setting at 42 psi is inside the usual 20-45 psi window. But those two checks prove the warning and emergency systems work. They prove nothing about the service brakes, which are what stop the truck. Passing the backup systems is not the same as passing the primary system.
Why applied-side is worse:
- It leaks when you are stopping. The released leak drains the tank while parked. The applied leak drains it during the stop, compounding with the air you are already spending.
- The failure can go total, instantly. A diaphragm that is weeping at 10 psi/min can rupture. Then that wheel has no service brake at all and the leak rate jumps.
- Run the clock. At 10 psi/min from 118 psi: about 6 minutes to the 58 psi warning and about 7.5 minutes to the 42 psi spring brake set. A single long descent, or a traffic jam on a ramp, gets you there - and the spring brakes will apply at whatever speed you happen to be carrying, on a tractor-trailer, without you choosing the moment.
3. What is the correct disposition of this unit, and what would you do if the driver had already left the yard before you measured it?
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Disposition: out of service, red-tag it, do not dispatch. The unit fails the applied leakage test by more than three times, and the fault is localised to a trailer wheel end.
If the driver has already left:
- Get him off the travelled road at the first safe exit - not 'the next town' - and have him hold the unit there.
- Tell him explicitly not to hold the brakes on and creep, and not to keep fanning to test them; each of those spends the air he needs.
- If he is on ice, snow or a grade, that instruction gets sharper, not looser: an automatic spring brake application on a slick downgrade is a jackknife risk.
- Do not tell him to cage a chamber to get home. Caging removes the parking and emergency brake on that wheel and does not fix a service-side leak.
The general rule this case is built around: a truck can have a perfectly functioning warning system and perfectly functioning spring brakes and still have no business moving. Those systems exist for the day the service brakes fail - not as a licence to run them into failure.