The pilot was carrying out a pre-flight inspection of the aircraft in preparation for an Instrument Rating flight test. He had selected the flaps down, and began operating the hydraulic hand pump to extend the flaps. After a few pump cycles, the right maingear collapsed and the pilot then noted that the gear selector was in the up position. It was suspected that the anti-retraction valve, which is designed to prevent gear retraction on the ground, was unserviceable. However, no fault was discovered. Tests determined that the gear could be unlocked if the hand pump was operated rapidly, and the pilot advised that he had pumped briskly on this occasion. The check list provided by the aircraft operator did not alert pilots to confirm the position of the gear handle before using the hydraulic hand pump. It was not possible to establish why or by whom the gear selector had been placed in the up position.