In a dual air brake system, what applies the trailer brakes if the trailer breaks away?

In a dual air brake system, when a trailer breaks away, the trailer brakes are applied through the use of a two-way check valve from the tractor air supply. This mechanism ensures that if the connection between the tractor and the trailer is lost, the trailer can still receive air pressure from the tractor's air system to activate its brakes.

This design is critical for safety, as it prevents the trailer from rolling freely and potentially causing accidents or damage. The two-way check valve isolates the brake system of the trailer, allowing it to function independently of the tractor once a breakaway event occurs, thereby helping to effectively stop the trailer.

Other options do not fulfill this role specifically. The emergency brake light serves a different function by indicating a fault or warning situation. A manual control lever involves driver operation and does not automatically engage in a breakaway scenario. Hydraulic brake fluid is not involved in the air brake system since air brakes use compressed air to function rather than hydraulic fluid. Thus, the reliability of the two-way check valve plays a crucial safety role in brake operation in case of a trailer breakaway.