What must always be checked before making changes to the brake system?

Checking the current brake cylinder pressure before making changes to the brake system is essential because it provides crucial information about the brake system's condition and its operational status. The brake cylinder pressure indicates how much pressure is being applied to the brake shoes against the wheels, which directly affects the braking capability of the train.

Knowing the current brake cylinder pressure helps determine if the brakes are functioning correctly before any adjustments are made. If the pressure is too low, it may indicate a problem in the brake system that needs to be addressed. Conversely, if the pressure is too high, making changes could result in brake application issues, risking safety. Proper evaluation of this pressure ensures that any changes made will not compromise the train's braking performance.

While environmental factors, weather conditions, and the train's speed are important for overall operational safety, they do not directly relate to the specific mechanics and operation of the brake system itself. The state of the brake cylinder pressure is a more immediate and relevant factor when considering modifications to the brake system.