The condition that causes vapour locking in a brake system is overheating of the fluid due to frequent brake application.
Although brake fluids are designed to work at extreme temperatures above 250 Degree Centigrade but still as the age of brake fluids goes on increasing, it starts loosing its properties and hence under high pressure and temperatures developed by applying brakes causes vapour lock.
The basic characteristics of a brake fluid is
* a high boiling point
* low viscosity
* compatibility with rubber and metal parts