Low Pressure Warning at Speed
A loaded three-axle dump truck is travelling at 70 km/h on a two-lane highway when the low-pressure warning buzzer and light activate. The gauge reads 58 psi and is falling about 5 psi every 10 seconds. The driver hears no obvious leak. The brake pedal feels normal at first, then becomes soft. At 42 psi the truck begins to slow without the driver applying the brakes. The outside temperature is -10°C. The nearest exit is 3 km away, but a safe pullout is 300 m ahead.
1. What is happening as the pressure falls, and what are the driver's immediate priorities?
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The low-pressure warning at 58 psi is correct because it should activate by 60 psi. The steady 5 psi per 10 seconds loss means the system is losing about 30 psi per minute, which is a serious leak or a compressor failure. At 42 psi the spring brakes are beginning to apply, which explains the truck slowing without a brake application. The driver's priority is a controlled stop at the 300 m pullout, not the exit 3 km away. The driver should keep steering, use engine braking and whatever service brake remains, and avoid pumping the brakes. Hazard lights should go on early. The driver should not release the parking control or try to override the spring brakes; the vehicle must be stopped before the spring brakes apply fully.
2. Why is it dangerous to continue to the exit or to keep pumping the brakes?
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Continuing uses up the remaining air and distance. As pressure drops, the spring brakes apply progressively and can stop the truck suddenly in a live lane; the driver may also lose the ability to modulate the service brakes. Pumping the brakes uses more air and speeds the pressure loss, and it does not fix a supply failure. If the compressor has failed or a line is frozen, pressure will not recover. Trying to reach the exit risks a stall or a locked-wheel stop in traffic. The 300 m pullout is the safer choice because it removes the vehicle from the travel lane before the spring brakes take full control.
3. After stopping, what should the driver check, and what criteria must be met before moving again?
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After stopping, the driver should set the parking brake, chock the wheels, and check for leaks at chambers, hoses, valves, drain cocks, and glad hands. In -10°C weather, look for ice in lines, especially at low points and the air dryer; warm frozen lines only with approved heat, never an open flame. Check the compressor belt, governor operation, and whether pressure builds at fast idle. The vehicle should not move until the system builds to governor cut-out, holds within leak limits with brakes released and applied, the low-pressure warning works, the spring brakes release fully, and all brakes apply and release. If pressure will not build or the leak cannot be found and repaired, the unit must be towed. Moving with a known pressure loss risks a runaway or a spring-brake stop at speed.