What was found
ON DESCENT FROM KPDX TO KBFI, RUDDER CONTROL LIGHT ILLUMINATED WITH HYDRAULIC SYSTEM PRESS READ ZERO AND HYDRAULIC QUANTITY DOWN TO 6 GALS. COMPLETED QR4 PROCEDURE (LOSS OF HYDRAULIC). CREW ABORTED DESCENT AND RETURNED TO KPDX. LANDED WITHOUT INCIDENT. AN INVESTIGATION IS ON GOING. AS SOON AS INFORMATION IS AVAILABLE IT WILL BE PROVIDED. MAINTENANCE FOUND SLOT CONTROL VALVE LEAKING. REMOVED, REPACKED AND REINSTALLED VALVE. THE OPERATIONAL AND LEAK CHECK WAS GOOD ON THE ENGINE RUN PER ALL APPLICABLE MANUALS. THE AIRCRAFT WAS RETURNED TO SERVICE.