If an airplane equipped with master cylinders and single-disk brakes has excessive brake pedal travel, but the brakes are hard and effective, what is the probable cause?

Excessive brake pedal travel, in the context of a system equipped with master cylinders and single-disk brakes where the brakes are still hard and effective, is typically indicative of worn brake linings. When brake linings wear down, they create a greater distance between the brake pedal and the brake components when the pedal is pressed. This increased distance leads to more travel in the pedal before the brakes engage, thus resulting in the excessive travel observed.

The condition of the brake system allows the brakes to still function effectively because, although there is more pedal travel, the mechanical advantage provided by the design of the braking system ensures that sufficient force is transmitted to the braking surfaces for effective stopping capability. In contrast, issues like mismatched brake components or improper fluid type in the system generally lead to poor braking performance or responsiveness, not simply increased pedal travel with a still effective brake. A faulty master cylinder seal would typically result in a soft pedal feel or potential loss of braking effectiveness due to fluid leakage rather than just excessive travel. Therefore, worn brake linings are indeed the most likely cause for the condition described.