A B767-300 IN CRUISE AT FL370 DECLARED AN EMER AND DIVERTED DUE TO L HYD SYS LOSS OF PRESSURE AND QUANTITY.

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WE RECEIVED A L HYD QUANTITY STATUS INDICATION ALONG WITH AN ILLUMINATION QUANTITY LIGHT ON THE OVERHEAD PANEL. WE CHKED THE L SYS QUANTITY AND SAW .48 RF WITH NORMAL SYS PRESSURE. WE RAN THE APPROPRIATE CHKLIST AND TURNED THE L SYS PUMPS OFF. WE CALLED SFO COMMERCIAL RADIO TO OBTAIN A CLRNC BACK TO ZZZ. I THEN INITIATED A L TURN TO THE N AND BROADCAST OUR INTENTIONS. APCHING 25 NM OFFSET; I BEGAN A DSCNT TO FL365. AT ABOUT THE TIME WE LEVELED OFF WE RECEIVED CLRNC DIRECT TO INTXN X DIRECT ZZZ AT FL370. WE CLBED BACK TO FL370 AND PROCEEDED DIRECT INTXN X AND AGAIN BROADCASTED OUR INTENTIONS. WE REPROCEEDED TO INTXN X AND REQUESTED TO HAVE THE CFR EQUIP TO STAND BY. WE CONFIGURED EARLY TO ACCOUNT FOR THE 1/2 RATE TRIM AND MADE A NORMAL APCH AND LNDG TO RWY 8L. WE CLRED THE RWY AND PROCEEDED TO GATE. I ASKED THE CFR TO MAKE A VISUAL INSPECTION FOR LEAKS WHILE WE TAXIED IN AND THEY RPTED NONE. WE SECURED THE ACFT AND TURNED IT OVER TO ZZZ MAINT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LOSS OF THE L HYD PRESSURE AND QUANTITY WAS A LEAK LOCATED AT OR NEAR THE UPPER RUDDER PWR UNIT. THE RPTR SAID THE EXACT FAILED COMPONENT IS UNKNOWN. THE RPTR STATED AN EMER WAS DECLARED.