When I built my 904 John Cope told me not to add a rear vent. It was also mentioned in the link Cope no longer vents at the rear.
When I added my trans brake I was told if I drilled the piston (I did) instead of the drum I wouldn't need to relocate the vent. Guess what? It blew fluid out the factory vent so the transmission had to come back out and partially apart to move it.
