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Hi,
@CJ_Boxster - Some systems are vented while others are not, with Modern ABS Systems, vented Reservoirs are being seen less and less. But, even the sealed systems can/do allow some air (along with it's moisture) to pass the Cap Seal - it's not a perfect seal.
I am totally familiar with the inner workings of a Master Cylinder and have certainly rebuilt many more than you have racked up years - not dissing you, just making an example, in fact I rebuilt a Girling MC for a friend of mine last weekend.
According to the SAE DOT 4 Brake Fluid absorbs between 1.5-2% moisture by weight annually, even in a sealed system. This is because the primary source of moisture invasion comes from the Flexible Brake Hoses at the Calipers and past the Piston Seals themselves. Fluid cannot pass, but minute amounts of moisture can.
Consequently, every 2 years the average system will contain about 4% moisture by weight and should be changed. This much moisture may not dangerously affect the Wet Boiling Point (though it may for some fluids), but 4% Moisture will attack the system components. Also, the SAE found that the moisture content was not linear throughout the system but was found in higher concentrations in the Calipers (because they are closer to the Flexible Hoses and Piston seals), while the Fluid in the Reservoir may have a lower moisture content. They use a Refractor to determine the Moisture content sampled from different parts of the system.
And, the worst place to experience Fluid Boil is in the Calipers because their effectiveness can be reduced to nothing as some people here have found...
Happy Motoring!... Jim'99
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