What was found
(CAN) DURING ROUTINE MAINTENANCE, WITH "B" PUMPS ON AND HYDRAULIC INTERCONNECT "OPEN" TO LOWER THE FLAPS, WHEN NORMAL OPERATING PRESSURE WAS REACHED IMMEDIATE LOSS OF HYDRAULIC PRESSURE WAS NOTICED ALONG WITH MASSIVE AMOUNTS OF HYDRAULIC FLUID GUSHING OUT OF THE MAIN WHEEL WELL. ON INVESTIGATION THE "A" SYSTEM AILERON POWER CONTROL UNIT INTERNAL FILTER RETAINING CAP SUFFERED CATASTROPIC FAILURE AND SEPARATION FROM THE THREADED PORTION OF THE CAP. THE "A" SYSTEM AILERON PCU WAS REPLACED AND HYDRAULIC SYSTEM SERVICED. SYSTEM FUNCTION CHECKED AND OPERATED NORMALLY.