this may sound off the wall but what type of coolant are you running in the cooling system?
I know in the automotive applications they run dexcool in thier cooling systems and and have seen it in some lifttrucks as well, dexcool has a tendency to eat up the gaskets. I know of many occasions i have seen tech bulletins about the corrosive nature of dexcool and the effect it has causing constant leaks from gasket sealing areas etc.

just a thought to pass along