What was found
(CAN) OLEO WENT FLAT, LOST ALL OIL AND NITROGEN AFTER A NORMAL LANDING. AIRCRAFT WAS FUELED AND CREW DEPARTED FOR 7 HOURS - UPON RETURN, OLEO WAS FLAT. LANDING GEAR HAD BEEN OVERHAULED AND INSPECTED FOR FIVE YEAR REQUIREMENT 07/12/99. UPON INSPECTION OF ASSY, UPPER SEAL 'O' RING WAS FOUND TO BE ROLLED AND ABRADED ALMOST 1/4 OF CIRCUMFERENCE. NEW SEAL INSTALLED. NOTE: THERE IS A SPECIFIC PROCEDURE FOR INSTALLATION IN COMPONENT MAINTENANCE MANUAL.