For us in our shop, when it comes to hydraulic cylinders, we tend to go on the philosophy of letting sleeping dogs lay.
Thing about hydraulic cylinders, some are very simple to rebuild. We do most of them ourselves in our own shop. Then there are those that are buggers. Some involve freezing things and some involve heating them to get the parts over other parts without distorting or breaking them. Those unfortunately we take to a specialty hydraulic shop. I say unfortunately because if it is a reasonably small cylinder (as opposed to something huge that would come off a dozer or excavator) it is sometimes just as cheap for us to buy a brand new cylinder over paying the labor and parts charge to rebuild one.
So as long as they are working, not leaking down or leaking externally, we tend to leave them alone.
The other danger of rebuilding them is if it happens to be a tricky one and the mechanic screws it up, you may end up bringing the tractor back to the shop to fix what they screwed up that was working perfectly well before they went into it. Trust me, it happens.
