Two Miles Into a Six-Mile Grade

The two failure modes look similar from the driver's seat and demand very different responses, so read the evidence:

• Pedal is firm and high. A supply problem - burst line, failed compressor, blown diaphragm - usually gives a soft, falling, or bottomed-out pedal, because you are pushing air into a leak instead of against a sealed system.
• Gauge at 68 psi and drifting slowly. A major leak would drop far faster and would trip the low-pressure warning at 60 within seconds, followed by the spring brakes. A slow drift under repeated applications is what a healthy system does when the compressor cannot quite keep up.
• Speed increasing despite a hard application. This is the decisive sign: the brakes are being applied and are not producing retardation.

That combination is brake fade - the drums and linings are heat-soaked, the lining coefficient has fallen off, and the harder you push, the hotter and less effective they get. Automatic slack adjusters do not prevent fade, and they do not correct for a heat-soaked drum.

Priority is to shed energy without adding heat or spending air:

1. Stop the hard continuous application. Holding the pedal down keeps generating heat and gets you nothing once the linings are faded. Release it, let the drums get whatever cooling the airflow provides.
2. Downshift progressively - 8th to 7th to 6th - and let the engine brake work at high rpm. Compression braking is free retardation that costs no air and adds no heat to the drums. Do not skip gears; you need each downshift to land without over-revving.
3. Use short, firm snubs rather than light drags. A hard application down to a lower speed, then fully off, then repeat, puts heat in over a shorter time and lets it dissipate between applications. A light continuous drag never lets the drum cool and is the classic way to cook a set of brakes.
4. Commit to the ramp early, not late. Half a mile at the speed you are carrying gives you a controlled entry. Waiting to see whether the curve is manageable means entering the ramp fast, or missing it altogether and taking the curve with faded brakes and 78,000 lb behind you.
5. Signal and move right deliberately - the box truck 150 feet back needs to see the manoeuvre, and a ramp entry at speed with no warning invites a rear-end collision.

Using the ramp: enter at the posted speed or below if you can, keep the steering wheel straight, and do not brake hard on the gravel - differential braking on a loose arrester bed can pull the truck sideways or tip it. Let the deep gravel do the work. Once the truck is slowing in the bed, hold the wheel straight and let it come to rest.

Once stopped:

• Set the parking brakes, chock the wheels, and keep the truck in gear as a backstop.
• Get out and look for fire at the wheels and the trailer bogie; heat-soaked drums and grease seals are a real ignition source.
• Let everything cool before touching anything. Do not 'just roll out and continue' - the brakes are still heat-soaked and the next application will fade faster than the last.
• Call for recovery and report the ramp use, so the ramp gets closed and cleared.

What went wrong: 45 mph in 8th on a seven-percent grade is too fast and too high a gear for 78,000 lb. The correct approach is to select the gear that holds the descent without service brakes - usually several gears lower - and to check the grade, the length and the brake condition before the sign, not after. The 20,000-mile slack adjuster interval also deserves a look on a truck that just faded this badly.