DURING DESCENT; A B737-400 FLT CREW NOTICES THE LEFT DUCT PRESSURE WAS LOW. WHEN THROTTLE ADVANCED; DUCT PRESSURE INCREASED. MAINT FOUND HIGH STAGE BLEED VALVE IN LOCKED-OUT POSITION AND UNDOCUMENTED.

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I FLEW A B737-400 FROM ZZZ1 TO ZZZ2. IT WAS THE FIRST LEG OF A 2 LEG PAIRING; THAT WOULD HAVE US FLY FROM ZZZ2 BACK TO ZZZ1 IN THE SAME ACFT. THE LOGBOOK WAS IN PROPER ORDER WITH NO MEL/CDL LISTED AND A PROPER AIRWORTHINESS RELEASE SIGN OFF. THE DEFERRED ITEM LIST IN THE FLT PAPERWORK SHOWED NO ENTRIES. DURING OUR DSCNT INTO ZZZ2 THE FO NOTICED ON THE OVERHEAD PANEL THAT THE L DUCT PRESSURE INDICATED ONLY 9 PSI. THE R DUCT PRESSURE INDICATED 25 PSI. I SUSPECTED A POSSIBLE HIGH STAGE BLEED VALVE PROB AS THE L DUCT PRESSURE RETURNED TO A VALUE ABOVE 20 PSI WHEN THE L THROTTLE WAS ADVANCED DURING THE LEVELOFF. I NOTIFIED ZZZ2 OPS TO RELAY OUR OBSERVATION TO THE MAINT CTL. AFTER ARRIVING AT THE GATE IN ZZZ2; I MET WITH THE 2 MECHS (ONE IS A TECHNICIAN TRAINER; VERY SHARP) AND EXPLAINED THE LOGBOOK WRITE UP. THE ENTRY I MADE WAS ON LOG PAGE X. THE MECHS BEGAN TROUBLESHOOTING THE L ENG BLEED SYS. WHEN THEY ATTEMPTED TO MANUALLY POS THE L ENG HIGH STAGE BLEED VALVE TO THE OPEN POS; THEY WERE UNABLE TO MOVE THE VALVE. UPON FURTHER EXAM OF THE VALVE; THEY OBSERVED THAT THE VALVE'S LOCK-OUT COLLAR WAS IN THE LOCKED-OUT POS AND BEING SECURELY HELD THERE BY ITS SET SCREW. I PERSONALLY OBSERVED THIS CONDITION WHEN THEY INITIALLY LOOKED AT THE VALVE. THE MECHS SUBSEQUENTLY UNLOCKED THE HIGH STAGE BLEED VALVE AND PERFORMED AN ENG RUN TO VERIFY ITS NORMAL OP. THE TECHNICIAN SIGNED OFF THE DISCREPANCY IN THE LOGBOOK. HIS PRESENCE WAS EXTREMELY HELPFUL IN RESOLVING THIS ISSUE. HE'S A VERY KNOWLEDGEABLE AND CAPABLE MECH. WE DEPARTED ZZZ2 JUST UNDER 4 HRS LATE. IT IS APPARENT THAT THE HIGH STAGE VALVE WAS LEFT IN THE LOCKED-OUT POS BY A PERSON OR PERSONS WHO WERE EITHER PERFORMING UNAUTH MAINT PROCS OR NOT PROPERLY FOLLOWING ESTABLISHED MAINT PROCS ON THIS ACFT. AS OF OUR DEP TIME FROM ZZZ2; I HAD NOT HEARD HOW LONG THIS ACFT HAD BEEN OPERATING IN THIS CONFIGN; BUT IT SOUNDED AS IF IT HAD BEEN A CONSIDERABLE NUMBER OF REVENUE FLTS. THE INDIVIDUAL'S ACTIONS; AND BY ASSOCIATION; SENIOR MGMNT'S POLICIES; PLACED THIS ACFT; OUR CREWS AND OUR PAX WELL-BEING AT RISK. I CONSIDER THIS EVENT; AND THE ENVIRONMENT WHICH ALLOWS THIS TYPE OF EVENT TO OCCUR; ENTIRELY UNACCEPTABLE AND A CONSIDERABLE SAFETY THREAT TO OUR OP.