What was found
WHILE TAXIING OUT PRIOR TO DEPARTURE CREW NOTICED A CAS, " NR2/ NR4 HYD LOW". ACFT WAS TURNED AROUND AND RETURNED TO THE FBO AND ENGINES WERE SHUTDOWN. INVESTIGATION REVEALED NR 2 HYD SYS HAD DRAINED OUT. LEAKAGE WAS NOTED AT THE FITTING END OF THE NR 2 HYD LOWER ACOUSTIC FILTER. O-RING, PN AS3208-08, WERE REPLACED AT BOTH END, SYS REPLENISHED AND LEAK CHECKED OK. ACFT RETURNED TO SERVICE.