What must be performed before proceeding after troubleshooting a brake problem?

Answer: B — class 2 brake test

Before proceeding after troubleshooting a brake problem, conducting a class 2 brake test is essential as it validates that the braking system is functioning correctly. This test involves applying the brakes in a controlled manner to ensure that they respond appropriately and that the system maintains acceptable pressure levels. The class 2 brake test helps confirm that any issues identified during troubleshooting have been resolved and that the train can safely operate without risk of brake failure. It checks for not only the response of the brakes but also ensures that all components are functioning cohesively, thus upholding safety standards. This procedure is crucial for maintaining operational integrity and ensuring safety for both the crew and passengers.