One Gauge Reads Zero While the Other Holds 105 psi
A driver starts a 2020 straight truck with a dual air brake system on a cold morning. The secondary gauge climbs to 105 psi; the primary gauge stays at 0. The low-pressure warning buzzer and light are on. The parking brake control can be pushed in only after several attempts, and the truck rolls forward when the driver releases the clutch in first gear. The service brake pedal feels firm and the truck slows when the driver presses it, but the primary needle does not move. The driver has a 40-mile route with two steep grades. The outside temperature is 18°F. The truck was parked overnight with the tanks drained. There is no obvious air leak and the compressor runs continuously. The driver has a load of bottled water and a 6:00 a.m. dispatch time. The secondary gauge holds steady at 105 psi while the engine idles.
1. What does the gauge split tell you about the dual air system, and what should be checked first?
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A split of 105 psi secondary and 0 psi primary means the two circuits are not both charging. Because the compressor runs continuously and the secondary builds and holds, the compressor itself is probably capable of making air; the primary circuit is either isolated, leaking, or not receiving air, or the primary gauge or sender is wrong. Check in this order:
- Verify the gauge. Compare with a known test gauge or the tank-mounted gauge if equipped. A failed sender can mimic a dead circuit.
- Check the primary tank and drain. Open the primary tank drain briefly with the system charged; if no air or only a trickle escapes, the tank is not receiving air. Ice in a line or a closed or blocked valve is plausible at 18°F.
- Look for a major leak or failed check valve. A failed check valve can let one circuit drain or prevent it from charging. Listen and use soapy water at the compressor discharge, governor, check valves, and tank fittings.
- Confirm low-pressure warning operation. It is already on; that is consistent with one circuit below 60 psi.
Do not assume the firm pedal means the system is healthy; the pedal may be operating only the secondary or front circuit.
2. Can the driver proceed because the service brake pedal still slows the truck? Explain the risk on the two grades.
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No. A firm pedal and some slowing do not prove a serviceable dual system. If the primary circuit is dead, the rear service brakes are not contributing normally, and the spring brakes may not be fully released or may apply unpredictably. The secondary or front circuit alone gives much less braking, and on two steep grades the truck can overheat the remaining brakes, fade them, or lose control. The parking brake control requiring several attempts and the truck rolling in first gear are warning signs that spring-brake release is compromised. The low-pressure warning is a legal out-of-service indicator. The correct action is to diagnose and repair before dispatch: warm the truck in a heated shop if ice is suspected, replace the failed check valve or line, and verify both gauges build to governor cut-out and pass a leak test. A 40-mile run with bottled water is not worth the risk.
3. If a test gauge shows the primary circuit really is at 0 psi, what does that mean for the spring brakes and for moving the truck?
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Spring brakes are held off by air pressure. If the primary circuit supplies the spring-brake release air on this truck, 0 psi means the parking brakes may not release or may drag. The truck rolling in first gear suggests they are not holding or releasing correctly. If the circuit is dead, do not move the truck; the spring brakes could apply suddenly when pressure drops further. Even if the truck can move, it has no reliable parking or emergency brake. Repair the primary circuit, then perform the parking-brake test: with the system charged, push in the control, confirm the truck does not move in low gear; pull it out and confirm the brakes hold. Only then release the truck for service.