problem with hi splitter gear

Hi i have a volvo fh12 v2, problem with a VT2514b 14sp with a retarder manual box, every gears goes in fine, but every time i try put into hi split it makes a grinding noise, even when im in neutral from lo splitter gear i clutch in split to hi then clutch out it grinds, sounds like its has a lose gear or its not engaging probably, or mayb the teeth on the gear is rooted, i have try pull the little valve on the top left side of the bell housing on top the clutch slave and change 3 o-rings on it as its was leaking, i have try plum work shop air into to make sure it has enough air, still same problem, any help would be appreciated and if any1 has full work shop manual for this box would be great as it looks like i might havta rip the box out and rebuild it, lets hope its something small.. thanks guys..