What likely needs to be replaced if the brake pedal slowly sinks to the floor while stopped?

When the brake pedal slowly sinks to the floor while the vehicle is stopped, this is often indicative of a failure within the hydraulic braking system. The most common reason for this behavior is an issue with the master cylinder.

The master cylinder is responsible for generating hydraulic pressure when the brake pedal is pressed. If there is internal leakage within the master cylinder, it can cause fluid to escape, resulting in a loss of pressure in the brake lines. This leads to the gradual sinking of the brake pedal because the pressure required to keep the pedal in place diminishes.

Replacing the master cylinder in this situation is crucial to restoring proper brake function and ensuring that the vehicle can stop effectively. Other components such as brake pads, the brake booster, or the brake fluid reservoir do not directly cause the pedal to behave this way when they are malfunctioning. Instead, they influence braking efficiency and feel but not the specific symptom of a sinking pedal.