As I read about these, the issue with the slaves is the seal breaking down. Could these be dismantled and a heavier duty seal fitted with additional grease added. If this is the case, I would imagine the plastic would serve better against the seal.
Also would it be feasible to have the slave mounted outside the box, with pipework going into the box? My only experience with slaves was donkeys years ago, on one fitted to the top of the gearbox. Replacement was literally a five min job, and 10 mins to bleed and test. It looked at you as you opened the bonnet. Was such a simple and easy design to work with.
EDIT: the slave used on the Celica engine is similar in design to the one I refer to
HERE. They have adapted it to suit but make mention of not being able to use a concentric slave cylinder that would fit (or perhaps they knew better lol)
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