Brake fluid is an element that is often “forgotten” and not replaced at the right time. It is hygroscopic (it means that it tends to absorb moisture in the air) and over time it degrades and loses the characteristics described above, developing as most evident effect the tendency to produce bubbles of water vapor with increasing temperature. When this happens you have the VAPOR LOCK EFFECT which involves the progressive lengthening of the hot brake lever. Cooling system with the effect disappears and the feeling with the lever returns to normal.
Note that if the brake fluid is in good condition (refer to the graph shown below), the reason for the raising of the temperature of the plant such as to enable the production of gas bubbles derived from the brake system component is not suitable or not comply, namely: the use of insufficient diameter discs, discs below the minimum allowable thickness, pads FADING … all elements that determine the components overheating.
Among the features that must have a brake fluid, two stand out in a particular way: the more can be incompressible and have a high boiling point. This is because it must completely transfer to the grippers the force applied to the brake lever and, when the braking system increases the operating temperature during operation, should not reach boiling point. If that happened it would occur the effect “Vapor Lock” caused by the production of gas bubbles that would result in the gradual increase of the lever stroke.