The brakes are out, and the brake pedal is going to the floor.
The complete brake system will need to be inspected to see if a major brake fluid leak from a component or metal lines is leaking. If there are no leaks and the fluid is full in the master cylinder, then the most likely cause is a master cylinder internal failure, which may require replacement. I recommend having a mechanic, such as one from YourMechanic, do a complete brake inspection.
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