Primetic, I really can not help you directly with determining what might be leaking or why. I would probably call Allison to discuss: 800-524-2303.
Also, I found a pdf file at an Allison website that discusses "Fluid Leak Diagnosis" in paragraph 2-9, page 37. I tried to copy and paste but it seems to be protected from copying. This file is for 3000 and 4000 trasmission products. But it seems to be generic enough to generally help. The pdf file can be found
Here
Paragragh 2-10 provides a small comment regarding the breather.
Hopefully someone with experience will respond with some more specific information. SteveG