If there's absolutely no sign of an external brake-fluid leak, but you lost that much fluid, it's possible you blew the master cylinder seal that faces into the power-brake booster.
An old master cylinder may develop internal rust or dirt build-up past the limit of it's normal piston travel.
Sometimes, pushing the pedal past it's normal travel, can damage the old piston-seals, by forcing them into this rust or dirt.
Unless it's unavoidable, whenever I do any brake work, I try not to push the pedal all the way to the floor. If I've opened the hydraulics, I'll gravity-bleed the system for awhile. Then I can usually use short pedal-strokes to pump up the brakes or seat the pads.

Happy Motoring, Mark

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DrDKW