The system is not going to be completely full of fluid, because for brakes to work, fluid leaves the master cylinder, when the brake is pushed, filling the wheel cylinders, and calipers full of fluid, which stops the car.
When the brake petal is released, the fluid travels back to the master cylinder. The wheel cylinders, and calipers will not be full of fluid and the car moves.
A cold system, when heat is introduced, will sweat causing condensation.
Its simple physics.
Why will a good looking brake line, actually break, because it rots from the inside out.
DOT 3 or 5 will work, if you have replaced every component, at our age you will not have to worry about it.
Real qualifications, I am a degreed engineer, and have worked in industry for over 30 years designing aircraft.