What indicates a malfunction in the hydraulic brake system during the brake check?

During a brake check, observing that the brake pedal moves down significantly over a span of five seconds indicates a potential problem within the hydraulic brake system. This phenomenon suggests a possible leak in the brake lines or issues with the master cylinder, leading to a loss of pressure in the system. A properly functioning hydraulic brake system should maintain a consistent pedal height when pressure is applied. If the pedal sinks, it can compromise braking efficiency, making this observation critical for identifying malfunctioning components.

Other indicators of a malfunction, such as warning lights or audibles, can suggest various issues, but they don't provide the direct physical feedback regarding the hydraulic system like the movement of the brake pedal does during the test. The loudness of the engine or fluctuations in the air pressure gauge are not relevant to the functionality of hydraulic brakes, further underscoring that observing the brake pedal behavior is the most direct indication of potential failure in this context.