General notes

Brake fluid is hygroscopic, i.e. it has the tendency to absorb water and moisture from the air. 

Fig 1: Brake Fluid Graph

Courtesy of VOLKSWAGEN UNITED STATES, INC.

If water has been absorbed, the boiling point will drop, i.e., 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.

Even the smallest amount on a seal or boot 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.

Fig 2: Sleeve Original Size And Swollen As A Result Of Contact With Mineral Oil

Courtesy of VOLKSWAGEN UNITED STATES, INC.

A = Boot - original size

B = Boot (swollen due to contact with mineral oil)

The above mentioned points require special rules for handling: 

Always keep brake fluid containers properly sealed. This is the only way of preventing contamination by oil, dirt, cleaning agents or moisture.

Brake fluid containers should be kept completely separate from oils (including hydraulic fluid) and cleaning agents to prevent any mixture of fluids or even filling of the brake system with an incorrect fluid.

Brake fluid is hygroscopic, i.e. it has the tendency to absorb water and moisture from the atmosphere 

Fig 1: Brake Fluid Graph

Courtesy of VOLKSWAGEN UNITED STATES, INC.

An increase in water content in the brake fluid causes its boiling point to drop. A reduced boiling point can cause vapor bubbles to develop as the fluid heats up under intense braking. In this condition the brake is likely to fail completely.

The color of the brake fluid becomes darker over a period of time. A dark brake fluid color is not an indication of its condition. The change in color is caused by chemical reactions and may occur within a short period (several weeks), particularly in vehicles equipped with ABS.

Even very small amounts of the above substances will transform structure and dimension of a sealing ring or sleeve enough to adversely affect brake function. The effects of a polluted brake system will not be noticed for several months until they have caused damage that requires extensive servicing, especially on vehicles installed with ABS.

Fig 2: Sleeve Original Size And Swollen As A Result Of Contact With Mineral Oil

Courtesy of VOLKSWAGEN UNITED STATES, INC.

A = Sleeve (original size)

B = Sleeve (swollen due to contact with mineral oil)

The above mentioned points require special rules for handling: 

Always keep brake fluid containers properly sealed. This is the only way of preventing contamination by oil, dirt, cleaning agents or moisture.

Brake fluid containers should be kept completely separate from oils (including hydraulic fluid) and cleaning agents to prevent any mixture of fluids or even filling of the brake system with an incorrect fluid.