Bleeding master cylinder in the car

sireland67 said

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.

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Sorry, but that just isn't how brake cylinders and calipers work. When you let up on the pedal, the pistons in the cylinders and calipers pull back since they aren't under pressure - no air involved, just brake fluid. I've never had a brake line "break" in 50 years of working on cars in continuous use. If a car sits for a long time and the system is no longer sealed, then moisture laden air can be introduced. If nobody replaces the lines there could be issues.

The idea that DOT 3 protects the lines by absorbing the moisture in a system, including at the surface of the reservoirs in the master cylinder, also seems suspect in that such DOT 3 would be considered to be contaminated and compromised.