brake fluid 
noun [mass noun]
Fluid used in a hydraulic brake system.The pistons have a thermal insulator and are drilled radially near the circumference in contact with the pads to limit the dissipation of heat to the hydraulic system's brake fluid....
- Problems will also arise if too much water mixes with the oil, transmission fluid, brake fluid, or antifreeze.
- The next step - previewed at auto shows and due in showrooms within five years - sends hydraulic brake fluid the way of the buggy whip by using smart electric motors at each wheel to apply the brakes.