Just confirmation of the diagnosis already given really. I had a master cylinder do that on a new van intermittently between 6k and 36k miles. I inherited from one of my bosses but he neglected to tell me of the faulty brakes where the pedal would remain solid despite standing on the brakes with no braking action. As it happened so rarely and "fixed" itself immediately until the next time, it took a while to diagnose.
I've also had a few master cylinders behave exactly as your does, suddenly goes to the floor when brakes are applied, either let it return by itself or pull up with your toes and it's fine again until the next time.
Not knowing when the next time is going to be and that it could be in an emergency situation, i would strongly advise not to mess about, you have ~1.5T of otherwise unstoppable missile without brakes.
You don't want to face charges of manslaughter by dangerous driving or similar because your brakes failed and you knew about them being faulty!
It won't go away, it will only get worse so get it sorted ASAP, not just for your sake but that of other innocent people who might face premature death if you can't stop!
