8 3/4 spitting gear oil out of breather vent

I have just taken my Dart out for another test drive to break in the new rear end after swapping the 7.25 rear end for an 8.75 rear end. Everything is new, including the breather vent, except for the housing of course. Not sure what year A body the housing came from, but it had a vent on it when I bought it that was damaged. The issue I am having is there appears to be gear oil spraying out of the breather vent. I have never encountered this before. I would like to know if anyone else has had this issue. What did you determine was the cause? What was your solution?
Thank you for your time and look forward to hearing what you have to say>