Having to change the master cylinder is a myth. If you get a proportioning valve that matches the year of the CAR, not the year of the rearend, then it will bolt right in and work perfectly.

And it is a great swap to do.....I just did it last week on my RS.

 

Just to add a bit more to the original topic of discussion, all of the 9 bolt rears came with posi. What's nice about the 9 bolts is that you can shim the cones after they wear, which is nice considering how much new parts cost for them.