As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PHL - EMERGENCY LANDING - FLT 0256 - EN ROUTE FROM PHX TO PHL AT CRUISE RECEIVED CLASS 1 MESSAGE FOR GREEN HYDRAULIC SYSTEM RESERVOIR LOW LEVEL, MANUALLY EXTENDED LANDING GEAR AND DECLARED AN EMERGENCY. MAINTENANCE FOUND HYDRAULIC LEAK IN LEFT WHEEL WELL WHERE PRESSURE SWITCH (10CE1) IS INSTALLED IN MANIFOLD. REMOVED AND REPLACED O-RING PACKINGS ON FLIGHT CONTROL PRESSURE SWITCH (10CE1), LEAK AND OPERATIONAL CHECKS WERE GOOD.