Cluth problems

1987 toyota 4x4pickup, manual transmission. Rebuilt transmission year ago, repl clutch, slave cylinder and master cylinder. Son took truck to wyoming for school, Has had numerous problems with truck out there, people keep blaming it on the altitude and extreme weather and dirt conditions. Clutch went out about 1 month ago, he replaced it bled system worked fine. Today it won't go in gear, pushes clutch but it won't activate so that he can shift. He thinks slave cylinder, it is less than a year old. Any suggestions on what to look at first. Also does altitude effect seals and gaskets. He tells me that all his seals and gaskets are leaking oil since he has moved out to WY. It wasn't doing any of this when he left 8 months ago. The engine only has about 60,000 miles on it. It is out of a 1992 truck.