Brake Fluid
Brake fluid is hygroscopic, that means it has the capability to absorb water and moisture from the air.
If water has been absorbed, the boiling point will drop, that means, during high braking temperatures the brake fluid may develop steam bubbles and cause the brakes to fail.
Over time, brake fluid will darken in color. Dark-colored brake fluid does not specify anything about its quality. The coloring occurs through chemical reactions and can occur after a short time (several weeks), especially in vehicles with ABS.
The rubber components of the brake system (rubber grommets, seals) do not tolerate mineral oil, gasoline, and cleaning agents!
Even the smallest amount on a seal or rubber grommet can cause the part to change and thereby affect the function of the brake system. The results of an unclean brake system first show themselves months later, causing increased repair costs, especially on vehicles with ABS.
A is the rubber grommet - original size
B is the rubber grommet - swollen through contact with mineral oil
It is implied by the previously mentioned points that:
Keep containers of brake fluid closed securely. Make sure oil, dirt, cleaning agents and air humidity cannot get in the container.
Store containers of brake fluid away from oil (even hydraulic oil) and cleaning fluids to prevent an accidental mixing of both fluids or even filling the brake system with the wrong fluid.
It is recommended to use Audi approved brake fluid.