A DC10 LOST #2 HYD SYS QUANTITY ON TAXI OUT. HYD RESERVOIR WAS PREVIOUSLY REPLACED AT SEA.

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ON DEC/XX/96; I CHANGED #2 HYD SYS RESERVOIR BECAUSE OF NO HEAD PRESSURE IN THE RESERVOIR. I REPLACED #2 HYD RESERVOIR PER MAINT MANUAL. I LEAK CHKED THE RESERVOIR AND FOUND NO LEAKS; SINCE THE PRESSURE AT THE TANK IS ONLY 45 PSI THE LEAK WOULD TAKE A LONG TIME TO SHOW. THE CAUSE OF THE LEAK WAS THE RELIEF AND SAMPLING VALVE THAT XXX MECHS FOUND LEAKING. THIS VALVE IS INSTALLED AT THE SHOP AND IS INSTALLED COMPLETELY AT THE SHOP. I SAW NO LEAKS AT THE TIME WHEN I MADE MY CHK. ALSO IT WAS RAINING THAT NIGHT AND RAIN WAS DRIPPING INTO THE TAIL CONE COMPARTMENT MAKING A LEAK VERY HARD TO FIND. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT AFTER CHANGING THE #2 HYD RESERVOIR IN YYY THE AIRPLANE WAS FLOWN TO XXX WITH NO LOSS OF #2 HYD QUANTITY. BUT ON DEP FROM XXX ON TAXI OUT THE #2 SYS LOST ITS FLUID. LEAK WAS FROM THE RELIEF AND SAMPLING VALVE. THIS COMPONENT IS NOT LINE REPLACEABLE AND IS INSTALLED ON THE RESERVOIR IN THE OVERHAUL SHOP. THE FAA HAS NOT CONTACTED THE RPTR.