A B767-300 WAS DISPATCHED IN NON COMPLIANCE WITH A L ENG HIGH STAGE BLEED VALVE RENDERED INOP PER THE MEL SPECIAL PROCS BUT THE PRESSURE REGULATOR SHUTOFF VALVE WAS THE ACTUAL VALVE RENDERED INOP.

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ON AUG/XA/99; I WAS WORKING FLT XYZ (ACR) FROM ZZZ TO ATL. A MECH ASSISTING ME MAY HAVE LOCKED OUT THE WRONG VALVE DURING THE COURSE OF AN MEL DEFERRAL. IT WAS NEAR THE END OF MY SHIFT; AND A NEW SHIFT HAD JUST COME ON DUTY. APPROX 10 MINS BEFORE DEP; THE FLC CALLED THE MAINT DEPT. THE L ENG 'BLEED' LIGHT WAS ON. DISCUSSING THE SIT WITH THE FLC AND MY LEAD; WE DECIDED TO DEFER THE DISCREPANCY PER THE MEL. THIS REQUIRED LOCKING CLOSED THE HP VALVE AND DEACTIVATING THE THRUST REVERSER. AT THIS TIME; AT LEAST 5 MECHS FROM THE ONCOMING SHIFT ARRIVED TO HELP. 1 MECH HELPING WENT TO WORK LOCKING OUT THE HP VALVE. IT NOW APPEARS HE MAY HAVE LOCKED OUT THE PRSOV; WHICH IS IDENTICAL TO THE HP VALVE. THE REVERSER WAS DEACTIVATED; ALL COWLING CLOSED; AND I SIGNED THE LOGBOOK FOR THE MEL DEFERRAL. WITH THE MEL BOOK IN HAND OPENED TO THE PROCS SECTION; I DOUBLECHKED WITH THE OTHER MECH THAT THE VALVE HAD BEEN PROPERLY LOCKED CLOSED AND WAS SATISFIED WITH HIS ANSWER. TO THE BEST OF MY KNOWLEDGE AT THAT TIME; ALL WORK WAS IN FULL COMPLIANCE WITH THE MEL AND MAINT MANUAL. NOT UNTIL SUBSEQUENT MAINT AT ANOTHER STATION WAS THE PROB DISCOVERED.