What was found
WATER PRESSURE ISSUE. RTFL. FOUND WATER TANK FULL AND FILL VALVE CLOSED. CREW REPORT, THERE WAS WATER PRESS ON GROUND, BUT NONE AFTER DEPARTURE. TANK DRAINED. LAV AND GALLEY DRAINS NOT AFFECTED. TRIED TO PRESSURIZE WITH ENGINES AND APU AIR, NO JOY. WATER SYS WILL PRESSURIZE AT GRD SVC PANEL. NEXT STEP IS CLEAN OR REPLACE AIR SHUTTLE VALVE & OR AIR FILTER. WATER SYS IS DRAINED. TROUBLESHOT TO REPLACEMENT OF AIR SHUTTLE VALVE AND AIR FILTER. REPLACED AIR SHUTTLE VALVE AND FILTER. FILLED AND PRESSURIZED WITH APU.