Hot Brakes on a Long Mountain Descent
You are driving a 3.5 tonne panel van towing a 1,200 kg braked box trailer down a mountain pass. The descent is 7 km long at an average 8% gradient, with a 40 km/h advisory limit for heavy vehicles and two hairpins. The van has an automatic gearbox with a manual mode and no retarder. About 2 km into the descent you notice a hot, acrid smell, the brake pedal travels noticeably further than it did at the top, and the van pulls slightly to the right under braking. There is a wisp of smoke from the right front wheel. Coolant temperature is normal, the ABS warning light is not on, and the trailer brake controller is set to its usual setting. There is a gravel pull-off 300 m ahead and a lay-by 1.5 km ahead. Traffic behind you is light. The road is dry and it is 14:00.
1. What is happening to the braking system, and why does gear selection matter more than the brakes on this descent?
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This looks like brake fade with fluid close to boiling, plus a right-front caliper that may be dragging. On a sustained 8% descent the brakes turn the combination's potential energy into heat far faster than the airflow can carry it away. Pads and discs overheat, friction falls away, and the fluid can boil so that gas bubbles make the pedal long and spongy.
A 4.7 tonne combination relying on the service brakes alone on 7 km of 8% gradient will always overheat them. The gearbox is the primary brake: selecting second or third in manual mode and letting engine braking hold the speed keeps the service brakes cool for the hairpins. The pull to the right and the smoke from one wheel suggest that caliper is not releasing, which keeps adding heat and will not be cured by a lower gear.
2. What do you do in the next few hundred metres, and what would make you refuse to continue?
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Stop at the gravel pull-off 300 m ahead, not the lay-by 1.5 km further on: the symptoms are already present and every extra metre adds heat. Slow gently on the brakes you still have, use the hazard lights, pull well clear of the carriageway and chock the wheels, because a very hot brake may not hold the vehicle on the gradient.
Do not pour water on discs or calipers: thermal shock can crack them and the steam can scald. Let everything cool, keep people away from the traffic side, and check the right-front wheel for heat and for weeping brake fluid. If the pedal is still long or spongy once cool, do not continue. That points to a hydraulic fault rather than simple fade, and the vehicle needs recovery.
3. How would you tell fade apart from boiled fluid and from a sticking caliper, and what should be checked afterwards?
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Fade recovers as the brakes cool, and the pedal comes back to its normal height and feel. Fluid boil typically gives a pedal that is normal when cold but sinks or feels spongy again as soon as the brakes get hot. A dragging caliper gives one wheel much hotter than the others, a pull under braking, and a burning smell that continues even when you are not braking.
A pedal that goes to the floor, or that will not hold the vehicle stationary, means hydraulic failure: do not drive. After any of these events, have the brake fluid replaced if it boiled, have the right-front caliper and slide pins checked, and have the trailer's brakes and breakaway system inspected before the next descent, because the same fault will return on the next long hill.