Leak Test Math on an Eight-Glad-Hand Set
You are assigned a triple set: tractor plus three 28-foot pups, eight glad hands in total. In the yard the system is charged to 112 psi, engine off, wheels chocked, trailer air supply valve pushed in. With the parking brakes released you read 112 psi at the start and 106 psi after 90 seconds. You then hold the treadle down hard and watch the gauge fall from 108 to 101 psi in 60 seconds. Later, bleeding the system down to check the warnings, the low-air buzzer sounds at 55 psi and the trailer air supply valve pops out at 45 psi.
1. Compute the leak rate for both tests and state pass or fail, using the correct limit for a combination vehicle.
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- Released-brake test: 112 minus 106 is 6 psi lost over 90 seconds. Rate = 6 / 1.5 = 4.0 psi per minute. The limit for a combination vehicle with brakes released is 3 psi per minute (a single vehicle gets 2). Fail.
- Applied-brake test: 108 minus 101 is 7 psi in one minute = 7.0 psi per minute. The limit for a combination vehicle with the brakes fully applied is 4 psi per minute (single: 3). Fail.
- Why the combination limit applies: the rule is written around the number of connections and chambers, and a tractor-trailer is the case it was written for. A triple set sits well beyond that: eight glad hands, a dolly, and three sets of chambers, so passing the number is the floor, not the goal.
- Verdict: this set does not leave the yard. Both tests fail, and the applied test fails by a wide margin.
2. The applied test leaks noticeably more than the released test. What does that narrow the search to, and how do you actually find the leak on a triple set?
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- A leak that appears or grows only when the pedal is down is on the service side. Air is only pressurized in the service circuit when you apply. Top suspects: a brake chamber diaphragm, a service line, or a service glad hand seal, somewhere downstream of the treadle valve.
- A leak present with the brakes released would point instead at the supply side: the emergency circuit, a compressor or governor issue, a reservoir, or a supply glad hand.
- Chase it by isolation, not by crawling around blindly. With the system charged and the engine off, close the shut-off valves at the rear of the first pup and watch the gauge. If the leak rate drops, the fault is downstream of that valve. Reopen, move to the next set of valves, and repeat until the leak appears and disappears across one coupling.
- Then confirm with soapy water at that glad hand and around the nearby chambers, and listen at the dolly, which is the most exposed and most-often-dragged part of the set.
- Do not sign it off on a guess. After any repair, re-run both tests from a full charge, because a leak you fixed on the service side can leave a supply-side leak you never looked for.
3. Are the buzzer and valve pop-out readings acceptable, and how do they weigh against the leak results in your decision to move the set?
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- Buzzer at 55 psi: acceptable. The low-air warning must come on before the pressure drops to 60 psi, so 55 is inside the window. It is late in the window, though, and worth noting on the report.
- Trailer air supply pop-out at 45 psi: acceptable but at the edge. The tractor protection valve should close and the red knob should pop out in the 20-to-45 psi band. 45 is the top of that band.
- Do these numbers rescue the set? No. The warning system working correctly tells you the protection devices are doing their job as the system bleeds down. It says nothing about how fast it bleeds down, and that is the failure here.
- Decision: do not move. A set losing 4 psi per minute with the brakes released and 7 with them applied will reach the warning range faster than you think on a grade where you are on and off the treadle, and the first symptom of a service-side leak is that the pups behind it stop braking.
- What would change the decision: both tests passing after a repair, run from a full charge, plus a visual confirmation of the repair point.