6 km Descent, Pedal to the Floor at the 4 km Mark
At 21:20 a loaded medium truck (gross 7,600 kg, air-over-hydraulic brakes, exhaust brake and retarder fitted) begins a 6 km descent on a mountain national road signed at 7% for most of its length. The road is still wet from earlier rain.
The driver selects 5th of 6 gears, leaves the exhaust brake on, and uses the foot brake in short applications at first. By the 4 km mark speed has climbed from 40 km/h to 68 km/h with the pedal pressed hard. The pedal now travels almost to the floor and feels spongy, there is a hot acrid smell, the low-pressure warning buzzer sounds intermittently, and the air gauge reads 0.45 MPa where it normally sits between 0.65 and 0.8 MPa. A car is close behind. A sign shows a 非常駐車帯 in 1 km.
1. Explain what has happened to the braking system. What does each symptom - the long spongy pedal, the smell, the buzzer, the gauge reading - tell you separately?
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Four symptoms, two overlapping failures.
The smell and the falling retardation are brake fade: the drums and linings are far past their working temperature, the friction coefficient has collapsed and the lining may be glazing or outgassing. The brakes are still being applied, they are simply no longer converting speed into heat fast enough.
The long, spongy pedal points to the hydraulic half of the system: boiled brake fluid (ベーパーロック) produces exactly this - a pedal that goes long and soft because gas, not fluid, is being compressed.
The buzzer and 0.45 MPa are the air half. Every application consumes air faster than the compressor can replenish it; below roughly 0.5 MPa the low-pressure warning operates and the spring brakes begin to apply on their own. That is a different failure mode from fade and it will not recover by easing off.
Together: overheating friction material, boiling fluid and a depleted air reservoir on a wet 7% grade at night. This is a developing runaway, not a brake that needs a rest.
2. State the immediate actions in order, and explain why simply dropping to a lower gear or relying on the exhaust brake at 68 km/h in 5th is not enough.
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Order matters.
- Warn: hazard lamps on; if there is a safe moment, a brief horn or headlight flash for the car behind.
- Get engine braking working: the retarder to maximum, exhaust brake confirmed on, and a progressive downshift 5th to 4th and then 3rd as speed permits, matching revs to avoid over-revving or locking the rear wheels on the wet surface.
- Apply the foot brake firmly and release, rather than riding it lightly - hard, short applications give the linings a chance to cool between them.
- Aim for the 非常駐車帯, stop, chock and let everything cool; the spring brakes will hold once air has rebuilt.
Why 5th gear is useless here: exhaust-brake and engine-brake force scale with engine speed and gear ratio. At 68 km/h in 5th the engine is turning slowly, so the retarding force is small - far below what holds 7,600 kg on a 7% grade. The gear that holds this descent is the gear that would climb it, typically 3rd. Waiting until 68 km/h also means each downshift is bigger and riskier on a wet road.
3. How should this descent have been driven from the top, and what should the driver do if the service brake fails completely?
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From the top, before moving: select the gear that holds the target speed on engine braking alone, engage the retarder and exhaust brake, and set the speed limit for the descent at 30–40 km/h - not at the posted limit. Then never let the speed build, because regaining control downhill is far harder than keeping it. Use the foot brake only in firm, separated applications to trim speed, glance at the air gauge each time, and check the mirror for the smell of hot linings at the first lay-by.
If the service brake is lost: progressive downshifting through every gear to first; retarder and exhaust brake to maximum; the parking brake applied gradually and in stages, never snatched, which would lock the rear wheels and spin the truck on a wet road; steer for a 非常駐車帯, 待避所 or the uphill side of the road; use the horn and hazard lamps continuously. Guardrails and snow poles are a last resort to scrub speed, chosen deliberately rather than hit by accident. Do not switch off the engine - it drives the compressor and the retarder.