Before coupling air hoses, what reduction should be made to the brake pipe?

The correct answer identifies that a 20-psi reduction in the brake pipe is necessary before coupling air hoses. This reduction is important as it helps to equalize pressure between the two trains or cars being coupled, ensuring that the automatic air brake systems function properly.

When a train or cars are coupled, it's crucial to have this pressure reduction to avoid potential backflow or miscommunication between the brake systems. A 20-psi reduction is typically enough to allow the systems to synchronize effectively without experiencing undue stress on the hoses or connections, which might occur with larger pressure changes.

In contrast, larger reductions or no reduction at all could lead to operational issues. A 30-psi or 50-psi reduction might cause excessive pressure difference that can create significant problems, while not having any reduction could result in improper engagement of the braking systems, potentially leading to safety hazards. Thus, the 20-psi reduction is a standard practice that ensures safe and effective operation during coupling.