Which of these is a sign of brake failure?

A spongy brake pedal is a clear sign of brake failure because it indicates a problem in the braking system. This can occur due to air in the brake lines, fluid leaks, or a malfunctioning brake component. Normally, when the brake pedal is pressed, it should feel firm and provide direct feedback to the driver, allowing for precise control over braking. A spongy feeling suggests that the brakes are not functioning properly, which can significantly impair the driver's ability to stop the vehicle safely.

In contrast, clear windshield visibility, increased engine power, and effective tire grip are not indicators of brake failure. These factors may pertain to other aspects of vehicle performance but do not reflect the condition of the brake system. Understanding signs of brake failure, like a spongy pedal, is crucial for maintaining safety while operating a commercial vehicle.