What was found
AC WAS AIRBORN ENROUTE, WHEN "L HYD QTY" EICAS MESSAGE APPEARED, FOLLOWED BY "L HYD SYS PRESS" EICAS MESSAGE. AIRCRAFT RETURNED, MECHANICS FOUND HYD LEAK AT O-RING INSTALLED AT LT HYD CASE DRAIN DELTA P INDICATOR. MECHANICS REMOVED AND REPLACED O-RING, SERVICED HYD SYS AND OPS CHECK SYSTEM, OK. AC RETURNED TO SERVICE @ 14:09Z, 4 HRS 14 MIN AFTER ARRIVING BACK. (K)