What was found
IN CRUISE FLIGHT AT FL400, DURING CRUISE CHECKLIST, NOTED HYD VOL WAS 20 PERCENT AND DECREASING. ONCE HYD VOL A CAS ILLUMINATED FOLLOWED APPROPRIATE CHECKLIST AND LANDED WITHOUT INCIDENT. DETERMINED HYD LEAK FROM RUDDER PCU UPPER UNIT HAD O-RING CUT ON PRESSURE LINE, REPLACED O-RING AND LEAKED CHECKED SATISFACTORY ALL WORK IAW MM 12-10-36 AND 32-40-00. ALSO, SERVICED HYD SYS AND BLEED BRAKES AND LT MAIN HYD SYS.