What is the primary purpose of closing MR end cocks in case of a leak?

The primary purpose of closing the main reservoir (MR) end cocks in the event of a leak is to prevent further loss of MR air. When there is a leak, the pressure within the main reservoir can drop, leading to decreased air supply for pneumatic systems that rely on it, such as brakes and door operations. By closing the end cocks, the system is isolated from the leak, preserving the remaining air in the main reservoir for continued operation of essential systems.

Addressing the other options, while speeding up the repair process and ensuring passenger safety are important considerations in the context of a leak, they are secondary to the immediate need to maintain air pressure. Similarly, allowing for emergency braking is a critical function of the pneumatic system but is contingent upon having sufficient air pressure in the main reservoir. Therefore, maintaining this pressure by closing the MR end cocks is the most essential first action in response to a leak.