04-03-12, 07:58 PM
As a rough guide, even a slightly binding rear brake will get above boiling point within a mile of leaving home.
and you don't even notice the 'slowing down' effect of that.
If the brake has been hot it is possible for the heat to boil the fluid, a quick bleed would confirm this, (the brake should be good after a bleed) if it is you need to find whats causing the brake to stick and rectify it before you ride it.
If you worried about falling off your bike, you'd never get on.