As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
Just prior to takeoff, the "R Hyd Press" EICAS Msg. came on and right hyd qty had dropped to "0.45RF". Aircraft returned to the gate. further investigation revealed the right aft outboard (#8) Brake flex line had ruptured. Line was removed and replaced as per M/M 20-10-10, and Ops checked as per M/M 32-41-00. Aircraft was released for return to service.