As you might be guessing by now Shimano never intended their STI units to be serviceable. They offer no service documents or replacement parts (other then non shift pod bits). Over the years I have read of a few who did take apart and reassembled their units. But the number who wanted to but couldn't or decided to not try is VASTLY greater then those who did. This is why about the only servicing ever mentioned is the degunking and relubing of older units. Even when Shimano actually had some internal failure issues (way back when) they would just warranty the entire pod/lever with a new one. Good luck. Andy.