If the line is at the top, then it is the vent line and it should be open.
Can you upload a picture to this question so I can see which line? I attached a picture for you to view. It contains the vent line at the top and yes, it does attach to a line to direct any fluid leak to the side of the transmission.
Let me know if you have any other questions.
Tuesday, October 26th, 2021 AT 11:32 AM