The pilot had completed his before takeoff checks, and was taxiing the tailwheel-equipped airplane onto the runway for departure. He added power and initiated a left turn to align the airplane with the runway, while attempting to stop the turn the right brake failed and the airplane ground looped striking the right wing on the gravel runway. The airplane sustained substantial damage to the right wing and aileron. An examination of the right brake master cylinder revealed that the right brake would not hold pressure and the O-ring within the master cylinder had dried out allowing brake fluid to leak past.