A B737-400 RETURNED TO THE DEP ARPT DUE TO #2 ENG BLEED INOP. FOUND PRESSURE REGULATOR AND SHUTOFF VALVE SECURED CLOSED.

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AS A RESULT OF A DUCT SPLIT DISCREPANCY ON A B-400 ACFT; I AND A CO-WORKER WERE TASKED TO R&R (REMOVE AND REPLACE) #2 ENG PRSOV (PRESSURE REGULATOR SHUTOFF VALVE). FOLLOWING THE R&R I SHORT-SIGNED THE PAPERWORK THAT I HAD DONE THE R&R ONLY. BECAUSE THE ACFT WAS STILL IN THE HANGAR; I VERBALLY COMMUNICATED WITH MY CO-WORKER THAT HE WOULD DO THE POST INSTALLATION AND OPS CHKS REQUIRED AS THE ACFT WAS TAKEN OUTSIDE. IN MY SHORT-SIGN; I ALSO FAILED TO SPECIFY IN WRITING WHAT I HAD COMMUNICATED VERBALLY; THAT PER MAINT MANUAL 36-11-15 PAGE 401-420 I HAD ONLY DONE THE STRICT R&R OF THE PRSOV AND NOT THE POST INSTALLATION TEST. I WAS COMPLETING THIS SHORT-SIGN AS THE ACFT WAS OUT DOING THE OPS CHK ENG RUN. AS I CAME IN ON MY SHIFT THE FOLLOWING EVENING I WAS TOLD THAT THE ACFT EXPERIENCED AN AIR-TURNBACK BECAUSE OF A FAILURE TO GET DUCT PRESSURE ON THE #2 ENG SIDE. SWING SHIFT CLAIMED THE MANUAL OVERRIDE ON THE PRSOV WAS SECURED IN THE CLOSED POS. I WAS CONCERNED BECAUSE; EVEN THOUGH I WAS NOT DIRECTLY INVOLVED WITH THE OPS CHK; IT WAS MY UNDERSTANDING THAT THE OPS CHK FOLLOWING INSTALLATION OF THE NEW PRSOV WAS NORMAL. MY CO-WORKER PERFORMED THIS TEST AND COMPLETED THE SIGN OFF OF THE DEFERRAL. I HAVE ASSISTED WITH THE R&R OF A PRSOV ONE OTHER TIME AND FELT COMFORTABLE PERFORMING THE TASK. I BELIEVE THIS SIT MAY HAVE BEEN THE RESULT OF MISCOM BTWN MY CO-WORKER AND MYSELF THAT A POST INSTALLATION TEST AND AN OPS CHK WERE NECESSARY FOR THE PRSOV. THE POST INSTALLATION TEST WOULD HAVE ESSENTIALLY LEAK CHKED THE NEW PRSOV AND RELATED AIR CONNECTIONS. I STILL DO NOT FULLY UNDERSTAND HOW THE PRSOV COULD BE SECURED IN THE CLOSED POS AND STILL PASSED THE OPS CHK. SUPPLEMENTAL INFO FROM ACN 710170: OBSERVED ENG RUN QUALIFIED TECH PERFORM PRSOV OPS CHECK. OBSERVED 40 PSI AT BELOW 50% N2. I WAS SATISFIED WITH THE RESULTS AND REQUESTED RETURN TO HANGAR. I PROCEEDED TO SIGN OFF DISCREPANCY. THE FOLLOWING SHIFT IT WAS MENTIONED TO ME THAT THERE HAD BEEN A PROBLEM WITH THE PRSOV VALVE AND THE ACFT MADE AN AIR TURNBACK. IT APPEARS THAT I MISUNDERSTOOD A PROC I FELT I UNDERSTOOD.