A B737-500 IN CRUISE AT FL310 DECLARED AN EMER DUE TO LOSS OF 'A' SYS HYD PRESSURE AND QUANTITY CAUSED BY A FAILED PWR XFER VALVE.

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I WAS CAPT ON ACR FLT XXX (RADIO CALL FLT XXX) LAX-SFO ON MAY/XA/00. JUST PRIOR TO LEVELOFF AT FL310; OVER RZS VOR; NOTICED 'A' HYD QUANTITY DECLINING. QUANTITY DECLINED TO 5%; FOLLOWED BY LOW PRESSURE WARNING LIGHTS. RAN IRREGULAR CHKLIST FOR 'A' HYD SYS PRESSURE LOSS. ELECTED TO CONTINUE TO SFO TO ALLOW TIME FOR CONTACT WITH DISPATCH AND MAINT; AND TO ALLOW FLT ATTENDANTS TO 'PREPARE' THE CABIN. WX WAS BETTER AT SFO. WE COORDINATED WITH DISPATCH AND MAINT; DECLARED AN EMER WITH ATC; AND ASKED FOR A HOLDING PATTERN ON THE ARR TO MANUALLY LOWER THE LNDG GEAR. AFTER SUCCESSFULLY LOWERING THE LNDG GEAR; I FLEW AN ILS/VISUAL APCH TO RWY 28R AT SFO. I INSTRUCTED THE FLT ATTENDANTS TO HAVE THE PAX ASSUME THE 'BRACE' POS FOR LNDG. I WANTED TO ENSURE THAT THE PAX WOULD BE IN A SAFE POS IF A GEAR COLLAPSED UPON LNDG. WE CLRED THE RWY; STOPPED THE ACFT; AND WERE TOWED TO THE GATE. UPON POSTFLT INSPECTION MAINT FOUND A BROKEN VALVE IN THE LNDG GEAR HYD LINES. THE 'A' SYS QUANTITY WAS ZERO. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LOSS OF 'A' HYD PRESSURE AND QUANTITY WAS CAUSED BY A SHUTOFF VALVE THAT ALLOWS THE PWR XFER UNIT TO EXTEND THE LEADING EDGE DEVICES WITH LOSS OF 'B' SYS.