Brake Fluid with reduced boiling point can create vapor by boiling in the wheel cyclinder. This can occur under adverse braking conditions, or with a stuck brake pad. With vapor buildup pressing on the brake pedal applies force to vapor and not the pad. The result may be sudden brake failure. The presence of water in the brake fluid decreases the protection against failure due to brake fluid boiling.
Manufacturers recommend regular brake fluid inspections.
Over a relatively short period of time brake fluid will absorb moisture from the air. To test a sample one or two drops of fluid are placed on the sample window. The clear sample cover is lowered into position and the image on the scale is viewed by pointing the instrument toward a light source and focusing the eyepiece. The test takes just a couple of seconds and can be run on samples from the master cylinder or individual wheel cylinders. Since this test requires no actual heating of the sample it can be performed on consecutive samples without waiting for the instrument to cool.