What happens if the brake drums are worn thin or turned too far?

The correct understanding of what happens when brake drums are worn thin or turned too far is that they build up heat too quickly. When brake drums lose material due to wear or excessive machining, they become less effective at dissipating heat generated during braking. As a result, overheating can occur more rapidly. This can lead to brake fade, where the efficiency of the brakes decreases significantly due to excessive temperatures that affect the brake components' performance.

The other choices do not accurately describe the consequences of worn brake drums. Improving braking efficiency is not associated with worn drums, as thinner drums typically lead to decreased performance. Worn drums do not help cool the brakes; instead, they hinder the ability to maintain optimal temperature control. Finally, reduced wear on brake linings would not be a result of thin or excessively machined brake drums; rather, it can contribute to uneven wear or even premature failure of the linings, as they may not engage properly with compromised drum surfaces.