Valve
 

Dave...I replaced my stock unit with an adjustable after market unit years ago. Very effective for AutoX...not sure these are available any longer and haven't noticed if it's in the catalog. I likely have the original instruction manual, etc. somewhere if anyone's interested in a possible source? Regards, Scott

As Dave mentioned, VW suppliers typically have them.

I just put a Wilwood adjustable valve in my car, as it's significantly non-stock, and that helped get the bias sorted-out.

Proportioning valve
 

Years ago when I was actively running races with POC, (race tires) I first replaced the valve with a VW Tee. But that was a Little too much. Rear brake lockup under trail braking. Then I replace the Tee with a small after market adjustable valve (I think it was from racers wholesale, and after a couple of adjustments, it was perfect, and I have not messed with it since.
Martin Jansen
Agoura Hills CA