A B737-200 RETURNED TO THE FIELD DUE TO LOSS OF HYD PRESSURE AND QUANTITY CAUSED BY AN INCORRECT GASKET INSTALLED ON THE AILERON PWR UNIT XFER VALVE.

Narrative

A MECH AND I WERE WORKING ON AUTOPLT PROB FOR ACR XYZ ACFT. THIS WAS A CONTINUING PROB AND AN AILERON PCU XFER VALVE HAD BEEN CHANGED ALREADY. MECH HAD TROUBLESHOT THE PROB TO AN AILERON AUTOPLT SOLENOID VALVE. WE GOT A COPY OF THE MAINT MANUAL FOR REMOVAL AND REPLACEMENT OF THE VALVE AND HEADED FOR THE ACFT. MY PART OF THE TASK OF REPLACING THE SOLENOID WAS LIMITED TO TAKING THE NECESSARY SAFETY PRECAUTIONS BY DEPRESSURIZING THE HYD SYS; PULLING CIRCUIT BREAKERS AND PLACING INOP TAGS ON THE PROPER SWITCHES AND SO FORTH ACCORDING TO THE B737 MAINT MANUAL. WHEN I CAME BACK DOWN; MECH HAD THE SOLENOID CHANGED. THERE WAS SKYDROL EVERYWHERE AND MECH WAS SOAKED; SO I SAID I WOULD FINISH UP BY LOCKWIRING THE MOUNT BOLTS. NEXT; I WENT AND PERFORMED AN OPERATIONAL TEST OF THE SOLENOID PER MAINT MANUAL. THE SYS CHKED OUT SATISFACTORILY. THEN I WENT DOWN INTO THE WHEEL WELL TO LOOK FOR LEAKS AND MAKE SURE THE AREA WAS CLR OF TOOLS. I DID NOT SEE ANY LEAKS AT THIS TIME. WHEN THE ACFT TOOK OFF AND AUTOPLT WAS SELECTED ON THE SOLENOID VALVE THAT NOW WORKED ALLOWED HYD FLUID TO THE XFER VALVE. THE XFER VALVE HAD AN INCORRECT GASKET ON IT AND ALLOWED THE HYD FLUID TO LEAK OUT AND THE ACFT RETURNED TO FIELD.