What was found
AFTER TAKEOFF, DURING FLIGHT A FAILURE OF THE NR 1 HYDRAULIC SYS OCCURRED. THE PRESSURE WAS LOST AND THE QTY INDICATION DROPPED TO ZERO. AFTER COMPLETING THE CHECKLIST, ACFT LANDED. EMERGENCY GEAR EXTENSION USED, AND EMERGENCY BRAKING, FOUND AN O-RING INSTALLED ON A PLUG FOR THE LT HYD MANIFOLD BLOCK HAD FAILED. THE PLUG WAS CHANGED AND TORQUED.