They can be rebuilt and it sounds like your O-rings are bad. The rings should only be a few bucks. The manual ones lock up a few seconds after the tractor is shut off and require a dump lever to release or open them up, whereas the automatic ones do not lock up when the tractor is shut off--which has caused a lot of cubs to take a quick, unexpected ride down a hill to the surprise of the owners who went in to get a quick drink!
Cub Cadet 123