Marco, you shouldn't have any leaks. I have taken an ARB apart, and it is very simple. You can get all the guts you need from ARB. Good news is that you won't have to mess with the pinion, which should allow you to simply reset the backlash and call it good. You will have to remove the ring gear.
Pull the locker apart, and see if you can clean up the piston ring. Its quite large and flat. You should be able to easily move it in and out. Stick it back together and just test it on the bench. If you still have issues, give ARB a call, and they can ship out out new stuff pronto.
The one I took apart had a gear failure and the locker had bits of metal in it. The piston ring was scored. New one from ARB and it was good as new.
Baby Beast 2- 1999 4Runner SR5
Baby Beast -1987 4Runner SR5-Gone, but not forgotten