As I said, it's all down to humidity and temperature. I know little about the physics of it, but I do know that warm air can hold more moisture than cold air. Therefore air having a high moisture content will be comparitively warm. If that air is in a contaner, which is then cooled, the air will lose it's abilility to hold as much water in suspension, resulting in some of it condensing on the walls of the container. What that container is made of, doesn't affect the principle.
This is not correct. Every time you brake, air is sucked into the reservoir through the vent hole to replace the same volume of brake fluid used to expand the callipers or cylinders. When the brake pedal is released, the fluid returns, and that same volume of air is vented to atmosphere. So this constant, 'breathing in and out' as you term it, does take place. Resulting eventually in a complete change of air in the reservoir. A trip on a humid day could result in that same humid air filling the air space in the reservoir. Followed by a cold night, moisture in that air will condense on the cold walls of the reservoir.
What fluid is used doesn't make any difference to the degree of condensation that occurs. The only difference is what happens to the water once it runs down into the fluid. In one it's absorbed. In the other it remains as a volume of water. Mike.