A PARKED B757 SLOWLY ROLLS OFF THE TXWY RUNUP PAD FOR PIT RWY 28R WHEN THE ACCUMULATOR LOSES PRESSURE WITH ALL HYD PUMPS OFF. THE ENGS WERE SHUT DOWN AS FLC WAS AWAITING THEIR EDCT TIME SOME 55 MINS AWAY.

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AT XY00 LCL; WE PARKED IN HOLDING PAD TO WAIT FOR AN XY55 EDCT. CAPT SET PARKING BRAKE; SHUT DOWN ENG WITH APU RUNNING. WE COMPLETED THE PARKING AND SECURING CHKLIST. HOLDING PAD LOCATED ON STEEP INCLINE; WITH A SHARP DROPOFF ON THE BACK SIDE. XY30 WE FELT THE ACFT BEGIN TO ROLL BACK WITH PARKING BRAKE SET. CAPT AND I BOTH NOTED PARKING BRAKE SET. CAPT TURNED ON ALL HYD PUMPS. CALLED FOR ME TO START #1 ENG. ACFT STOPPED. WE STARTED #2 ENG. IT WAS OBVIOUS THE AIRPLANE HAD COME VERY CLOSE TO ROLLING OFF BACK SIDE OF THE INCLINE. R SIDE OF ACFT WAS LOW AND NOSE GEAR WAS ELEVATED OFF THE GND. CAPT BROUGHT PWR UP TO APPROX 50%. IT WAS OBVIOUS ACFT COULD NOT BE MOVED UNDER ITS OWN PWR. CAPT EARLIER COMMANDED ALL PAX TO REMAIN SEATED AND DO NOT EVAC ACFT. WE HAD 2 TUGS AND 1 TRACTOR DISPATCHED TO US. PAX STAIRWAYS ARRIVED AND ALL PAX WERE DEPLANED. R MAIN WAS ALMOST OFF INCLINE; SUNK IN DIRT. RAMPS WERE PLACED IN FRONT OF R MAIN. THEN; WITH 2 TUGS AND 1 TRACTOR AND ALMOST FULL PWR; WE WERE ABLE TO MOVE ACFT FORWARD. I DO NOT UNDERSTAND WHY; WITH PARKING BRAKE SET; THE ACCUMULATOR FAILED TO MAINTAIN BRAKE PRESSURE. I BELIEVE THE CAPT'S QUICK ACTION OF PLACING HYD PUMPS TO ON PREVENTED AN ACCIDENT BY ALLOWING THE ELECTRIC PUMPS TO PRESSURIZE THE SYS. THE WHOLE EVENT TOOK PLACE IN PROBABLY NO MORE THAN 10 SECONDS (FROM ROLLING TO STOP). ACFT DAMAGE APPEARED MINIMUM; WITH SMALL AMOUNT OF DAMAGE UNDER R ENG COWLING. SUPPLEMENTAL INFO FROM ACN 399401: PARKING BRAKE WAS STILL UP. TRIED TO RESET IT TWICE WITH NO SUCCESS. TOOK PAX AND FLT ATTENDANTS OFF THROUGH FORWARD R DOOR VIA TRUCK STAIRS. ACFT TYPE B757. CALLBACK CONVERSATION WITH RPTR ACN #399401 REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THE POSTFLT INSPECTION REVEALED A BAD ACCUMULATOR. IT LOST PRESSURE AT THE RATE OF 1000 LBS IN 2 MINS WHILE THE ACCEPTED RATE IS 50 LBS PER HR. THE PARKING CHKLIST CALLED FOR ALL 4 HYD PUMPS TO BE PLACED IN THE OFF POS IF NOT AT THE GATE; WHICH IS WHAT THE CREW DID. THE CHKLIST HAS NOW BEEN MODIFIED TO HAVE THE CREW LEAVE 1 ELECTRIC HYD PUMP ON IN THIS TYPE OF CIRCUMSTANCE. THE VICE PRESIDENT OF SAFETY FOR THE AIRLINE IS WORKING WITH PIT ARPT AND ATC IN AN ATTEMPT TO PRECLUDE A REPEAT OF THIS EVENT. THE ARPT IS PLACING A BARRIER NEAR THE EDGE OF SAID PARKING AREA. THE ACFT HAD NEARLY SLID OFF THE STEEP SIDE OF THIS AREA INTO A PARTIAL RAVINE; IT HAVING A 3-4 DEG DOWNSLOPE ON THE CEMENT PORTION. THE CAPT STATED HE ACTED INSTINCTIVELY IN TURNING ON THE PUMPS AND STARTING 1 ENG.