In an emergency caused by a trailer tank failure, what can be used for emergency braking?

Using the trailer air supply valve for emergency braking in the case of a trailer tank failure is correct because this valve applies air pressure to the trailer's braking system. This is particularly vital in emergency situations where immediate stopping power is required to prevent accidents or further system failure. The trailer air supply valve can effectively engage the trailer brakes, which is crucial when there is a failure in the tank system, as it allows for a quicker response from the braking mechanism.

The foot brake and parking brake may not provide the necessary emergency stopping power specifically for a trailer equipped with air brakes, especially if the air system is compromised. The hand brake, while useful under normal conditions, may not always provide the immediate braking force needed in an emergency scenario with a trailer setup. Hence, leveraging the trailer air supply valve is the most effective and appropriate response to loss of control in this context.