What was found
WHILE HOLDING SHORT OF THE RUNWAY, THE FLIGHT CREW RECEIVED THE MASTER CAUTION WARNING & A HYD VOLUME LOW A CAS MSG. THEY NOTICED THE A SIDE HYD QTY INDICATING 16 PERCENT & DECREASING AT A RATE OF APPROX 1 PERCENT EVERY 2 - 5 SECONDS. THE APPROPRIATE CHECKLIST WAS FOLLOWED & THE ENGS WERE SHUT DOWN. A CREW MEMBER EXITED THE ACFT TO ENSURE NO HAZARDOUS CONDITIONS EXISTED. HE FOUND THE LEAK COMING FROM UNDER THE LT PYLON ADJACENT TO THE FUSELAGE. A LARGE POOL OF FLUID WAS ON THE GND & A DRIP TRAIL FOLLOWING THE ACFT. THE ACFT WAS TOWED TO A MX FACILITY FOR INSPECTION & REPAIR. THE LEFT HYD PRESS LINE UNION BETWEEN THE LT HYD OVERLOAD PRESS VLV & PRESS TRANSDUCER O RING FROM THE UNION PROTRUDING OUT. THE O RING, PN NAS1612-10A, WAS REPLACED, SYS SERVICED & LEAK CHECKED WITH NO LEAKS OR OTHER DEFECTS NOTED.