[rovertenrod]

Not quite true. As brake fluid is Hygroscopic (it absorbs water) it's density changes. The watery stuff sinks to the low areas. It also heats up and as we know hot fluid will rise in cool fluid (density change). This is why the bottom of a master cylinder rusts ( internally) and why we should replace the fluid regularly.

 

[rovertenrod]

Brake systems aren't sealed. Without a vent the level can't change as linings wear.

 

[rovertenrod]

I've had at least three vehicle that had boiled the fluid due to water content. Worst was the Mk 3 Astras. The stiffeners under the bonnet would direct water to a drain hole directly above the reservoir where it would get in through the vent. Once the fluid becomes saturated the calipers have almost pure water in them. Ford Cargos (pre Iveco) used to have this happen regular if you didn't change the fluid at the recommended interval ( 2 years). There was a time when the Ministry of Transport was bringing this into the Test. Lots of garages bought the test kit and they never brought the rule in. Shame I say! Some cars this is the only check it gets all year.