What was found
(CAN) SHORTLY BEFORE DEPARTURE, RAMP CREW REQUESTED MAINTENANCE TO INSPECT A MAIN WHEEL TIRE. LOCAL MAINTENANCE ARRIVED AND CONFIRMED FLAT TIRE ON THE NR 1 AND NR 3 MAIN WHEEL ASSEMBLIES. INSPECTION OF THE MAIN WHEEL ASSEMBLIES REVEALED THAT 1 OF THE TIRE INFLATING VALVE CAPS WAS LOOSE AND THE OTHER ONE MISSING. FURTHERMORE, BOTH VALVE CORES APPEARED TO HAVE BACKED OFF BY APPROXIMATELY 1 TURN. ALL 4 MAIN WHEEL ASSEMBLIES WERE REPLACED AND THE ACFT RELEASED FOR SERVICE. MAIN WHEEL ASSY SN 2624. MAIN WHEEL ASSY, SN 2457.