What might be a major effect of not maintaining proper brake fluid levels?

Maintaining proper brake fluid levels is crucial for the safe operation of a vehicle's braking system. When the brake fluid level is low, it can lead to a significant reduction in the hydraulic pressure needed to engage the brakes effectively. This diminished pressure can cause the brakes to fail entirely or perform poorly, resulting in a dangerous situation where the driver is unable to stop the vehicle as intended. Brake failure, therefore, is a critical consequence of neglecting brake fluid maintenance, which can pose severe risks not only to the driver but also to others on the road.

The other options do not accurately reflect the consequences of low brake fluid levels. Increased fuel efficiency is unrelated to brake fluid levels and is typically influenced by factors such as tire pressure and engine health. Similarly, reduced vehicle speed is not a direct result of low brake fluid; instead, it may occur if a driver is cautious due to a known braking issue but is not a guaranteed effect of neglecting brake fluid maintenance. Hence, brake failure stands out as the significant and dangerous outcome of not maintaining proper brake fluid levels.