Originally Posted by
Polaris OBark
DOT fluid can absorb water from the air, even if it is just sitting on the shelf.
The hydraulic system is sealed, but contaminants can seep in, fluid can seep out, especially at the piston interfaces. Unlike inner tubes (which are also semi-permeable), there is no net positive pressure when the brakes aren't being squeezed, so there can be a gradual exchange with atmospheric water vapor (or whatever is on the surface of the pistons).
It happens with mineral oil as well, but mineral oil has much less of a propensity to absorb water molecules from the air.
Makes sense, PEX tubing does the same thing so radiant flooring systems have air bleeding built into them. Aluminum lined PEX is supposed to avoid that. I wonder if that would work with brake lines.