The amount of brake fluid that actually moves when you press the brake is trivial and with the forces involved it could be as thick as grease and still work mos

The amount of brake fluid that actually moves when you press the brake is trivial and with the forces involved it could be as thick as grease and still work mostly fine. Unless you actively mix it with water (like you do antifreeze) you are going to have a hell of a time getting glycol based brake fluid to absorb enough water to freeze at any earthly temperature. Maybe if you ran a system without the cap on the reservoir for decades in a very humid climate you could do it but even then I think the odds are long at best. I vote condensation.