If your vehicle is showing brake pedal goes all the way to the floor, here are the most likely causes and the parts you'll need to fix each one.
If brake work was recently performed, air may not have been fully bled from the hydraulic system. Because air compresses easily compared to fluid, the pedal will feel spongy and can travel all the way to the floor.
The internal seals inside the master cylinder can wear out or tear over time. When this happens, brake fluid bypasses the piston internally instead of pushing out to the wheels, causing the pedal to sink without any visible external leaks.
A severely rusted metal brake line or a dry-rotted rubber brake hose can burst under pressure. This results in a rapid loss of brake fluid and a complete loss of hydraulic braking power.
The square-cut seals inside the brake caliper can fail, allowing brake fluid to leak out around the piston. This loss of fluid and pressure will eventually cause the brake pedal to drop to the floor.
Old brake fluid absorbs moisture over time, which significantly lowers its boiling point. During heavy or prolonged braking, the fluid can boil and turn into a gas, causing the pedal to suddenly go to the floor.
Rarely, the internal dump valves inside the anti-lock braking system (ABS) module can get stuck open due to debris or mechanical failure. This allows fluid to bypass internally, mimicking a failed master cylinder.