What was found
C HYD QTY EICAS MESSAGE FOLLOWED BY A C HYD PRESSURE EICAS MESSAGE. PERFORMED CHECKLIST AND RETURNED TO DEPARTURE FOR A FLAPS 20 LANDING. EMERGENCY DECLARED. SERVICED CTR SYS HYD. PERFORMED LEAK CHECK ON CTR SYS HYD. FOUND LEAKING AT THE LCCA FILTER COVER. REMOVED COVER AND FOUND BOTH O-RINGS ARE BAD. R & R LCCA COVER O-RING IAW MM 27-11-15-024-001, 27-11-15-424-015. PERFORMED LEAK CK IAW MM 27-11-05-024 & MM 27-11-15.