Brake fluid absorbs moisture. That is why brake system needs to be completely flushed and bled at least every two years on street driven.
Other wise, it will invite rust on your master, wheel cylinders and hard lines.
The fluid freezes a bit that's why the brake pedal gets harder to push on cold freezing mornings until its warmed up after driving for a while.
This happened to me and it was scary not to have brakes when you need it!.
Thanks, I'll let it warm up on cold nights then/ I am broke right now but in january I am going to put a new clutch in, have the brakes flushes, Fully tuned up/ All new fluids, Etc..