I discounted those possibilities since the OP said that the pedal went all the way to the floor. On ice, there would be no braking, but the pedal would remain at the normal firmness. With the pedal blocked, it wouldn’t go as far down as it normally does.
Modern systems are dual-diagonal, so LF and RR share one sub-reservoir and RF & LR share the other.
I had a master cylinder fail on me, but that was in very hot weather. How full was the brake fluid reservoir? How long since the brake fluid was flushed? I also suspect the extreme cold was a factor - the master cylinder may be just fine.
Instead of holding the pedal to the floor, pump it!
But no system works when the fluid is gone. I have fixed this problem on three different cars. It is experience talking here. It is not the only possible cause but it is one of them. All brake systems are not identical either.
Good advice to pump em up. It works more often than not.
Okay, you are right that not all are the same and I certainly haven’t seen or worked on all cars. All of the cars that I have had have two separate reservoirs inside the one single looking one, even though there is only one small round fill cap and the reservoirs combine at the top. Clearly you have found examples contrary to this. I would have thought that didn’t meet regulations.
I once had a 78 Plymouth Volare that had reservoir separation. However, when the hydraulics ruptured on one side, all braking was lost. The reservoirs were separate, but there was an (until then) undetected fault in the master cylinder that still caused both to be drained.
One boss/tech I worked with had a rule that he said served him well,I will repeate it, this guy insisted on changing the rubber hoses in any brake system that was giving odd hard to diagnois symptoms. You might want to change yours out, they don’t last forever, and can cause hard to diagnois conditions when they either flex or collaspe.
Might also be the brake booster, in which case the brakes would still work, but would require a lot more force and could make it feel like the brakes stopped working.