Narrative:

There had been a continuing hydraulic leak in the #2 engine/pylon area that appeared to be coming from the area of the pump. The other mechanic and I spent several hours cleaning and inspecting the hydraulic system and engine areas. The source seemed to be the pump. We ordered a new pump and installation hardware and went to the hotel. When the pump arrived later I assisted in removal and replacement of #2 hydraulic pump. The pump was removed by another mechanic and I was to change the fittings and replace the o-rings. At this point I must not have installed one of the o-rings. The other mechanic installed the new pump. We did an engine run for the operations check and the pump functioned properly. The leak check was difficult due to the amount of residual fluid in the engine area. The other mechanic (engine run qualified) did not believe it was safe to run the engine with the cowls open. We did a couple more leak check runs and then would open the cowls to check for leaks but this was difficult due to the excess fluid in the engine compartment. We did one last engine run for 30 minutes to check for fluid consumption. The fluid level did not decrease. We opened the cowls and checked the area again and once again it was hard to determine leakage. We closed the engine up and cleaned the area and did the documentation on the repair. On a later flight the aircraft lost hydraulic fluid and had to shut the system down.I believe that I had a moment of inattention during the process and missed reinstalling an o-ring. I do not remember not installing the o-ring and believed I had properly installed them all when I gave the pump to the other mechanic to install on the engine.increased attentiveness on my part. I should have double checked the installation.

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Original NASA ASRS Text

Title: Air carrier maintenance technician reported a post maintenance loss of hydraulic quantity was likely due to a o-ring installation error.

Narrative: There had been a continuing hydraulic leak in the #2 engine/pylon area that appeared to be coming from the area of the pump. The other Mechanic and I spent several hours cleaning and inspecting the hydraulic system and engine areas. The source seemed to be the pump. We ordered a new pump and installation hardware and went to the hotel. When the pump arrived later I assisted in removal and replacement of #2 hydraulic pump. The pump was removed by another mechanic and I was to change the fittings and replace the o-rings. At this point I must not have installed one of the o-rings. The other Mechanic installed the new pump. We did an engine run for the operations check and the pump functioned properly. The leak check was difficult due to the amount of residual fluid in the engine area. The other Mechanic (engine run qualified) did not believe it was safe to run the engine with the cowls open. We did a couple more leak check runs and then would open the cowls to check for leaks but this was difficult due to the excess fluid in the engine compartment. We did one last engine run for 30 minutes to check for fluid consumption. The fluid level did not decrease. We opened the cowls and checked the area again and once again it was hard to determine leakage. We closed the engine up and cleaned the area and did the documentation on the repair. On a later flight the aircraft lost hydraulic fluid and had to shut the system down.I believe that I had a moment of inattention during the process and missed reinstalling an o-ring. I do not remember not installing the o-ring and believed I had properly installed them all when I gave the pump to the other Mechanic to install on the engine.Increased attentiveness on my part. I should have double checked the installation.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.