What happens if one of the service brake circuits of a dual-circuit compressed air-braking system leaks when the vehicle is in motion?

In a dual-circuit compressed air-braking system, if there is a leak in one circuit, air will escape from that circuit. This loss of air pressure can greatly reduce braking force because one circuit is no longer working properly. To prevent the problem from affecting the entire braking system, a four-way protection valve is used. This valve keeps the intact circuits supplied with air and prevents their pressure from dropping below a safe level. However, the protection valve does not supply extra air to the leaking circuit; it only isolates the problem to avoid total brake failure. That’s why answers 1 and 2 are correct—they describe the loss of braking in the leaking circuit and the role of the four-way valve in protecting the remaining circuits. This knowledge helps you understand the safety measures designed into modern commercial vehicle braking systems.