What was found
PILOT REPORTED FUEL CROSSFEED VALVE APPEARS TO BE LEAKING AFTER BEING CLOSED. AFTER CENTER TANK FUEL WAS USED, RE-BALANCED LT AND RT TANKS. THEN RIGHT TANK FUEL QUANTITY DROPPED COMPARED TO LEFT TANK QUANTITY. FUEL WAS GOING BACK INTO CENTER TANK. FUEL WAS BALANCED PRIOR TO DESCENT. AT GATE LT WING TANK SHOWED 3.81 RT TANK 3.16 . THE AIRCRAFT WAS GROUNDED. MAINTENANCE CONFIRMED BY TROUBLESHOOTING THAT THE FUEL COMPENSTOR WAS FOUND TO BE OUT OF SERVICE, REMOVED AND REPLACED THE COMPENSATOR IAW AMM 28-41-21-401 REV 125. OPS CHECK WAS GOOD.