After selecting the landing gear down the pilot reported that the hydraulic pump motor made an unusual sound for about 20 seconds, then stopped. He attempted to pump the gear down manually but was unable to obtain a safe indication although all landing gear legs appeared to be in the down position. Because of failing light he decided to land as soon as possible, and when flares had been arranged made an approach for runway 10. During the landing roll the pilot applied heavy braking which was evident due to scuffing and a flat on the right main wheel tyre tread. This caused the right main gear leg, which had not locked down, to rotate rearward around the hinge point and collapse. The aircraft slewed to the right, coming to rest in a shallow ditch alongside the runway. A corrosion hole was found in the left main gear door hydraulic line which had allowed hydraulic fluid to escape overboard.