A MAINT SUPERVISOR AND MAINT CONTROLLER REPORT REALIZE LATER THEY DID NOT HAVE THE AUTHORITY TO DEFER AND CAP OFF THE HYD PRESSURE LINES FOR A B777-200 'H' FITTING IN THE LEFT WHEEL WELL CEILING TO THE MAIN GEAR STEERING ACTUATOR.

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DURING WALKAROUND; TECHNICIAN FOUND A LEAKING HYD LINE IN CEILING OF THE L WHEEL WELL. FOUND PREVIOUSLY REPAIRED H-FITTING LEAKING. COULDN'T STOP LEAK AND FITTING WAS GALLED ONTO LINE; CAUSING THERE TO BE TOO MUCH TUBING REQUIRED FOR REMOVAL FOR INSTALLATION OF ANOTHER H-FITTING. TUBE PART #XXX WASN'T STOCKED IN OUR SYS. AFTER DISCUSSING DIFFERENT POSSIBILITIES WITH THE MAINT CTLR; WE DECIDED TO CAP BOTH ENDS OF THE PRESSURE LINE AND THE LEAKING LINE AND PLACARD THE MAIN GEAR STEERING INOP TO PRECLUDE A LENGTHY DELAY. I HAD A TECHNICIAN CAP THE LINES AND THE DEFERRAL PROC WAS COMPLIED WITH FOR MEL. I PLACARDED THE MAIN GEAR STEERING INOP PER THE MEL. ENGINEERING AUTH WAS ISSUED THE DAY AFTER THE NEXT RON VISIT. AFTER REVIEWING ALL PERTINENT DOCUMENTATION; I CONCLUDED THAT I DIDN'T HAVE PROPER AUTH TO CAP THE TUBE AND DISPATCH THE ACFT IN THIS CONFIGN. FURTHER IN THE FUTURE; I WILL CONSULT WITH OUR FIELD ENGINEERING DEPT IN THESE MATTERS. COM: THE TECHNICIAN ON THIS JOB WAS NEVER INFORMED THAT HE WASN'T WORKING UNDER PROPER AUTH ENGINEERING AUTH. I TOLD HIM WHAT TO DO; SO HE TRUSTED THAT I HAD TAKEN THE APPROPRIATE MEASURES TO ASSURE THE PAPERWORK WAS CORRECT. IN THE PAST; IT WAS COMPANY PRACTICE TO CAP HYD LINES AND DEACTIVATE SYS. I HAVEN'T DONE IT IN YRS AND DON'T RECALL THE SPECIFICS OF PREVIOUS INCIDENTS. SUPPLEMENTAL INFO FROM ACN 793326: THEY SUPPLIED FLUID ONLY TO THE STEERING ACTUATOR. THE LEAKING TUBE WAS CAPPED.