Descending a long downgrade can trigger a runaway even when your brake system is in good working order. Here’s why.
The treadle valve, under the pedal, sends only a small air signal. It activates a relay valve on each axle, which applies the brakes at a pressure that varies with the signal pressure.
Relay valves don’t send any pressure at all into the system until they reach a certain level, their crack pressure. And, especially if aftermarket valves have been used to replace original equipment valves, the crack pressures might not be the same on all axles.
Consequently, a light, steady braking pressure down a long grade often causes only three or four of the five axles to apply. Because those three or four axles are doing all the work, they get too hot. Once one or more axles overheat, the remaining axles may be unable to shed enough energy to keep the vehicle speed under control, even with maximum treadle pressure.
• Shift to the lowest gear that allows the truck to run at the posted truck speed limit.
• Apply the engine brake at the highest setting that allows the speed limit to be maintained.
• As needed, apply the service brakes using the “snubbing” method. This means applying enough pressure – usually 10 to 30 psi – to cause the truck to slow gradually. That amount of air pressure will open all relay valves and apply all the brakes.
• After the truck has lost 5 to 6 mph, release the brakes, which enhances their cooling.
• As you regain the speed limit, reapply the brakes hard enough to reach the recommended pressure range. Repeat this procedure until the road levels. n