What was found
AIRCRAFT GROUNDED - AFTER PUSHBACK, WITH ONE ENGINE RUNNING, PARKING BRAKE WAS ON WITH ECAM MESSAGE DISPLAYED. AIRCRAFT ROLLED FORWARD, BRAKE PRESSURES READ ZERO ON TRIPLE INDICATOR. ONCE ENGINE WAS SHUT DOWN, TRIPLE INDICATOR RETURNED TO NORMAL. SERVICED PARKING BRAKE ACCUMULATOR PER AMM 32-44-11. [REPEAT DISCREPANCY] PRIOR TO PUSHBACK, BRAKE ACCUMULATOR WENT TO ZERO BRAKE PRESSURE. REMOVED AND REPLACED BRAKE PRESSURE RELIEF VALVE PER AMM 32-44-15, OPERATIONAL CHECK GOOD, NO LEAKS NOTED.