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Can anyone explain exactly what I need?



Get rid of that valve in the picture! One of the advantages of 4 wheel disc is the lack of the need for valves. You may not even need the Wilwood adjustable if the rear piston size and pad compound give you the brake balance you want.

Start with no valve at all, then on a wet/damp WIDE stretch of road or better a big parking lot - do panic brake stops at 50 mph. If the rear of the car consistently wants to break loose and come around on you add the Wilwood valve and adjust it till the car stops trying to swap ends. Don't go too tight or the car will not want to turn because the rear is acting like an anchor. Great for trailer brakes, bad for performance handling.