What was found
(CAN) AFTER PUSH BACK, AND AFTER START CHECKS COMPLETED (BOTH ENGINES IN FEATHER). WHEN PARK BRAKE RELEASED, AIRCRAFT MOVED FORWARD DESPITE CAPTAINS & CO-PILOT FOOTBRAKES APPLIED, EMERGENCY/PARK BRAKE APPLIED GENTLY TO STOP AIRCRAFT. AIRCRAFT THEN TOWED BACK TO STAND.ADDITIONAL INFO:ENGINEERS COMPLETED CHECKS & FOUND NO DEFECT WITH SYSTEM, ALL HYDRAULIC LEVELS CHECKED & FUNCTION OF SYSTEM PROVED SATISFACTORY.ONE WEEK PREVIOUS THE HYDRAULIC ISOLATION MOD WAS COMPLETED WITH VARIOUS HYDRAULIC PIPES REPLACED.