As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ACFT 327 EN PUSHED OFF FROM THE GATE AND TAXIED TO THE RUNWAY FOR TAKE OFF. AT THAT TIME THE NR 1 HYD PRESSURE INDICATOR WAS READING ZERO PSI. FLIGHT 4645 RETURNED TO THE GATE AND LINE MX WAS CALLED. THE MAIN HYD INDICATOR WAS CHANGED AND THE PROBLEM WAS CORRECTED WITHOUT FURTHER INCIDENT.