Got any duff flex hoses in the system? Not leaking enough to cause a major leak, but just enough to let the gear unlock. I used to work on Aztecs back in the seventies, but I can't remember what hold the legs up. If it's definitely not the power pack, then a minor leak might be the cause. If your hyd flex hoses are original (which they probably are) it would be a good idea to get them pressure tested anyway, although that's probably a job for the annual check. Good luck anyway.