B757-200
Maintenance Airframe; Maintenance Powerplant
Situational Awareness
Aircraft Equipment Problem Critical; Inflight Event / Encounter Other / Unknown
General Flight Cancelled / Delayed; General Maintenance Action
Aircraft; Human Factors; Procedure
Helped replace number one Engine Driven Hydraulic Pump (EDP) per Aircraft Maintenance Manual (AMM) 29-11-05. Idle ran engine approximately ten minutes; no leaks noted.
While accomplishing ETOPS Pre-Departure Check; I noticed the number one engine cowling was wet with hydraulic fluid. I opened the [fan] cowl and found hydraulic fluid puddled inside cowl.  I requested help to run the engine.  I noted number one engine EDP(engine driven pump) leaking at mating surface of pump housing. I proceeded to replace number one engine EDP using AMM 29-11-05.  I am the one that removed and replaced the pump and tightened all [hydraulic] lines. After I was done; my partner went through the checklist [asking]: Did you tighten all four nuts; three lines and Cannon plug?   During flight; the Left-Hand (LH) Hydraulic System Quantity (QTY) and pressure dropped to zero. Left engine drive pump and electric switch turned to 'off' position after pressure loss.   Upon arrival; the Mechanic found LH EDP supply line loose and back-shell loose.
An ETOPS B757-200 lost left system hydraulic fluid in flight after having the engine driven pump replaced.