I have a 98’ model Volvo a35c with a transfer case puking oil out of the vented fill cap. I’ve replaced the cap but it’s still doing it. It’s not cross mixing fluids from the transmission either. Where could the pressure be coming from or why is it doing this. Also I filled it up until the fluid came out of the check plug.
Transfer case, I think you are talking about what we call the dropbox, the actuation of the difflocks is with air so you are getting air bypassing and straight into the dropbox.
item 9 is the seal probably causing you problems