Originally Posted by
rosefarts
All my brakes are mineral oil. The one DOT system I used was a SRAM that didn’t even make it to its one year mark, since i disliked it then sold it so quickly.
I am having a hard time understanding why it would absorb water. The system is sealed. Air can’t enter and neither can water. So how does this happen?
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.