What was found
AT 1 PLUS 30 INTO A FLIGHT, ESTABLISHED IN CRUISE, THE LT HYD LOW PRESS ANNUNCIATOR ILLUMINATED. THIS WAS SUBSEQUENTLY FOLLOWED BY A RT HYD LOW PRESS ANNUNCIATOR LIGHT. THE MAIN SYS PRESS GAUGE INDICATED A 'ZERO'. A NORMAL LANDING WAS MADE USING BACK-UP SYSTEMS FOR LANDING GEAR BRAKES, THRUST REVERSERS, AND FLAPS. LANDING WAS MADE AT JAX. AN O-RING SEAL, LOCATED ON A FITTING SITUATED BETWEEN THE MAIN SYS HYD SYS FILTER (AFT), AND A CHECK VALVE, FAILED. REASON FOR FAILURE UNKNOWN. THIS ALLOWED ALL PRIMARY HYD SYS FLUID TO BLEED FROM SYS. THE O-RING WAS REPLACED, SYS SERVICED, AND ACFT RETURNED TO SERVICE.