Originally Posted by
TheThomaswastaken
As af guess, in a closed system, cold fluid would take less room than warm. So when it warmed, it would create extra pressure in the brake lines.
It is not a closed system, the lines are open at the reservoir. So any excess fluid due to expansion would return to the reservoir.
The lines are open so that the system can make up any losses of fluid, only when the master cylinder pistons move and seal the openings in the reservoir can it be called a closed system. One piston actually moves before the other in our system and many others because of a spring between them.