Primary Gauge Falls on Route 17
Your 2018 Kenworth T680 tractor is loaded to 78,000 lb GCW. After a normal stop at a light on Route 17, you notice the primary (rear) air gauge falling from 120 psi to 90 psi in about 25 seconds while the secondary (front) gauge holds at 120 psi. The engine is at idle and you hear no obvious leak. You are 8 miles from the terminal on a two-lane road with a narrow shoulder. The low-pressure warning buzzer sounds when the primary gauge reaches 60 psi, and the spring brakes apply at 45 psi. Your co-driver says to keep moving because the secondary circuit is still good and the warning is just a reminder. You have a loaded trailer and a 6 a.m. delivery.
1. What does the gauge behavior tell you about the dual air brake system, and is the secondary circuit actually protecting you here?
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The primary and secondary circuits are separate so that one failure does not empty the other. The secondary gauge holding at 120 psi shows that separation is working, but it does not mean the truck is safe to drive.
A drop from 120 to 90 psi in 25 seconds is a major leak in the primary circuit. The compressor cannot replace air that fast. As primary pressure continues down, you lose rear service-brake performance, and when system pressure reaches the spring-brake range, the parking/emergency springs will apply. The low-pressure warning at 60 psi is functioning, but it gives you only seconds of warning, not a repair margin.
The secondary circuit gives you limited front braking only. It does not restore the rear brakes and does not keep the spring brakes released. Treat the secondary gauge as evidence of circuit isolation, not as permission to continue.
2. State your go/no-go decision and the first actions you would take.
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No-go. Do not try to cover the last 8 miles.
- Signal and stop as soon as you can get fully off the travel lane; a narrow shoulder is still better than a live lane, but do not stop where the trailer blocks traffic.
- Set the parking brake, place the transmission in neutral or park, and chock the wheels if you are on any slope.
- Turn on four-way flashers and set out warning devices as required.
- Keep the engine off or at idle and do not repeatedly pump the brakes; you will only use up the remaining air faster.
The risk is not just losing stopping power. If pressure falls to the spring-brake range while you are moving in traffic, the brakes can apply suddenly and lock the wheels. That is a much worse outcome than stopping where you are.
3. Which test would you perform to confirm the leak, and what would you look for?
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With the system fully charged, engine off, wheels chocked, release the parking brake and note the gauge for one minute. Then apply and hold the service brake for one minute and note the drop again.
- If pressure falls with the brake released, the leak is on the supply side: a chamber, hose, fitting, valve, or the compressor/governor circuit.
- If it falls mainly with the brake applied, the leak is in the service side: a brake chamber diaphragm, hose, fitting, relay valve, or treadle valve.
Listen at each wheel and use soapy water on fittings and hoses. Given the speed of this drop, the vehicle should be repaired before it moves under its own power. A leak this fast is an out-of-service condition even if the secondary gauge still looks normal.