As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING TAXI OUT ON THE GROUND PRIOR TO DEPARTURE WITH BRAKES SET RT HYD QTY EICAS MESSAGE WITH QTY READING 0.0. AC RETURNED TO GATE(BTB) AND MTX GROUNDED AC. MTX FOUND R BRAKE METERING VALVE LEAKING AT BODY. REMOVED AND REPLACED R METERING VALVE PER AMM 32-41-02, SERVICED R HYD SYSTEM PER AMM 12-12-01, OPS AND LEAK CHK -OK PER AMM 32-41-00. AIRCRAFT REMAINS ETOPS 180. AND RETURNED AC TO SERVICE.