As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: YELLOW HYD SYS RSV LOW. AIR INTERRUPTED BACK TO PHL. NO EMERGENCY DECLARED. THE AIRCRAFT WAS NOT OVERWEIGHT FOR LANDING. THE FLIGHT LANDED WITHOUT INCIDENT. MAINTENANCE FOUND BAD O-RING/FITTING CRACKED AT ALT BRAKE DISTRIBUTION DUALVALVE. REPLACED FITTING AND O-RING PER AMM 32-43-14.