As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) ON PASSING FL100, NR 1 HYD SYSTEM QTY WAS NOTICED TO BE LOW AT 50 PERCENT. WHILE MONITORING OVER THE NEXT FEW MINUTES IT WAS NOTICED THAT THE QTY WAS REDUCING. AT APPROXIMATELY IT WAS DECIDED TO TURN BACK TO BASE. ON RETURN SLOWED THE ACFT DOWN AND LOWERED GEAR AND FLAP BEFORE LOSS OF FLUID. COMPLETED QRH CHECKLIST AND NORMAL CHECKLISTS IN DESCENT. MADE AN UNEVENTFUL LANDING. BALANCE RELIEF VALVE FOUND LEAKING AND REPLACED.