The master cylinders on our old trucks are open to the atmosphere. There is a tiny hole in the top of the filler cap. That is where the water gets in (humidity in the air).
More modern vehicles have closed brake systems. They have a collapsible accordion-like diaphragm in the filler cap so that as fluid level drops when you push on the brakes, the diaphragm lowers down allowing the master cylinder to push fluid to the wheel cylinders without drawing in humid atmospheric air into the system. Once you release the brake pedal, fluid returns to the master cylinder and the diaphragm returns to the original position.
