Brake Fade on a Six-Percent Grade with the Engine Brake Turned Down
You are driving a 78,000 lb GCW five-axle combination down a 6% grade that runs four miles. The posted truck speed is 45 mph. You entered the grade at 45 mph in 9th gear at about 1,700 rpm. To hold 45 you have been applying the service brakes firmly for about three seconds every 20 to 25 seconds. Near the three-mile mark the truck starts gaining speed, now 52 mph, even though you are braking the same way. The pedal now travels noticeably farther before you feel resistance, and you smell hot friction material. The air gauge holds at 110 psi, there is no warning light, and the engine brake is on but in its lowest of three settings because the driver before you said the highest setting “is annoying on this road.” The shoulder is narrow and the next runaway truck ramp is 1.5 miles ahead.
1. Explain what is happening to the brakes and why pedal travel changed. Why does the steady air pressure rule out an air-system leak?
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This is brake fade from overheating. The drums and linings have absorbed a large amount of heat from repeated applications on a long grade. Two things happen at once:
- Mechanical: the drums expand with heat, so the pushrod has to travel farther before the shoes contact the drum. That is why the pedal travels farther before you feel resistance - the extra travel is taking up drum growth and normal slack, not compressing air.
- Friction: lining material loses friction coefficient at high temperature, so the same pedal force produces less stopping force. That is why the truck gains speed even though your braking pattern has not changed.
The steady 110 psi gauge and the absence of a warning light tell me the air system is not leaking and the compressor is keeping up; if this were an air loss, pressure would be falling and the low-pressure warning would eventually come on. The hot friction smell confirms a thermal problem. Fade can partly recover as the brakes cool, but severe overheating can glaze linings, crack drums, and damage seals - so it is not a “drive it out” situation.
2. What should you have done before and at the top of the grade? Be specific about speed, gear selection, and engine brake use.
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The descent should have been set up before the truck rolled onto the grade:
- Check the brakes and adjustment during the pre-trip and at the top of the grade; a brake that is already near its adjustment limit has less heat capacity to spare.
- Choose the gear before the descent, and choose it by the rule of thumb that you should be in a gear no higher than the one you would need to climb the grade. At 78,000 lb on 6%, 9th gear at 1,700 rpm was evidently too tall given the braking being used.
- Slow to the posted truck speed or below at the crest, not after the speed has already built. Braking to control a speed you already have is much harder on the brakes than entering slow.
- Put the engine brake on its highest setting. The previous driver's “annoying” comment is irrelevant - the loud setting is the one that does the most retardation and saves the service brakes.
- Snub rather than ride: brake firmly down to about 5 mph below the safe speed, release, let the speed build back, and repeat. That gives the drums cooling intervals instead of continuous heat input.
3. You are now at 52 mph and still gaining, with the ramp 1.5 miles ahead. Give your plan, including whether to use the parking brake, and how you would park if you stop on the shoulder.
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Immediate actions: put the engine brake on its highest setting now, downshift aggressively (one or two gears, watching rpm), and apply firm continuous service braking to get the speed back down. Continuous firm braking is appropriate here - the “snub, don't ride” rule assumes the brakes are not already faded. If speed still climbs despite maximum retardation and firm braking, use the runaway ramp; it exists for exactly this and a 1.5-mile window is enough time to commit.
Do not pull the yellow knob at 52 mph. Spring brakes apply at full force with no modulation. Locking the rear wheels on a grade can cause a jackknife or complete loss of steering control.
If you stop on the shoulder: do not set the spring brakes against smoking-hot drums - the linings can fuse to the drum or the drum can crack as it cools unevenly. Put the transmission in gear, chock the wheels, and let the brakes cool. After cooling, re-check pushrod travel, look for cracked drums and grease/oil from melted seals, and do not continue until the brakes are functional and correctly adjusted. Expect the fade to have left the adjustment suspect.