Brake fluid.
I had it in my rat-bike Triumph coming down the Stella Alpina - the fluid was old, contained too much water (brake fluid absorbs moisture over time), lowering the boiling point, and when it was worked hard enough and got hot enough the fluid boiled - hey presto, brake fade.
I changed it, problem solved. As a rule of thumb if the brake fluid looks the same colour as if you had pissed it you'd think about going to a Doctor then it's time to change it....
Any excuse for a gratuitous rat-bike Triumph pic!
