How To: Brake on long downgrades

The problem   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 ...