The residual valve must come out of the front system.
-The job of the R-valve is to keep around 10 psi in the lines.When installed in drum systems it used to keep the wheel cylinder rubber cups slightly ballooned to keep them from leaking. Now if retained in the disc system, this pressure will tend to keep the pistons from retracting, and the pads will remain in contact with disc.The pistons are normally pulled back into the calipers by the square section O-rings by a process that is called seal-retraction. This force is quite small, compared to the residual valve pressure.This may be okay if they were working underwater, but not so good in atmosphere.The constant dragging could cause all sorts of problems.Just use your imagination.
