Brake fluid: the level does not indicate the water content

Useful to know

Glycyl brake fluids such as DOT 3, DOT 4 and DOT 5.1 are hygroscopic. HELLA explains that the increased water content lowers their boiling point. [1]

The normal level in the tank is not a measurement of this property. The inspection and replacement follow the vehicle prescriptions and an appropriate method. Don't mix different designations under the "bigger number means universally better" rule. Compatibility and specification are important, not just the number on the packaging. When complaining about the pedal, the aging of the liquid is just one of the possible topics to check.