As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(ELCU) ENROUTE, CREW RECEIVED THE EICAS MESSAGE (C HYD PRIM 1) AND THE LOW PRESSURE LIGHT ILLUMINATED. FLIGHT RETURNED AND LANDED WITH NO FURTHER INCIDENT. MAINTENANCE REPLACED THE CENTER NR 1 ACMP AND THEN THE TIME DELAY RELAY. ON A SUBSEQUENT FLIGHT, THE PROBLEM REAPPEARED. THE ELECTRICAL LOAD CONTROL UNIT WAS REPLACED.