I really don't know too much about brake systems but as a graduate chemist I know quite a bit about the handling of hygroscopic fluids. Actually in some chemical reactions the slightest trace of moisture can cause a fire or explosion.
That aside, I woould like to bring something foward from the chemistry lab.
We use a tube that is packed with a highly active desiccating agent..silica gel or anyhdrous calcium chloride. This tube would be connected to the air vent on the master cylinder. Any air entering the cylinder would than be stripped of any moisture by the desiccant. It's that simple.
BTW, these desiccants are available with an 'indicating' agent in them that turns from blue to pink when the moisture capacity is exceeded.
Any comments from the mechanics in the group?