On downwind after the first take-off following a scheduled servicing the pilots were unable to fully extend the landing gear. After all efforts to lower the gear by normal and emergency methods were unsuccessful, the pilot in command carried out a safe landing with the gear retracted. Part of the work carried out during the servicing included the replacement of 'O'-ring seals in the main gear up-lock valve. During re-assembly of this valve, a ball bearing, whose function was to direct fluid to the actuator, was omitted. When the system was activated, insufficient pressure could be generated to fully extend and lock the gear.
