Worn disc brake pads. It's normal for brake fluid levels to drop as brake pads wear. A leak in any brake hydraulic circuit ...

Air in the brake system can make your brake pedal feel spongy and vague. You should flush the fluid every two or three years, ...

Your brakes are a hydraulic system ... If you let your brake pads wear so thin that the brake fluid level drops too low in the master cylinder reservoir where you fill the system with fluid ...