I am with Alvin, not totally needed, but then it doesn't hurt anything either. I use disk/drum with a proportioning valve, no residual valves, and a adjustable porportion valve on the rear. Mine would lock the rears up to quick, so the adjustable valve limits the pressure to the rear. Its harder to push the pedal being manual, but stops just fine. I am not sure the valve I am using up front does much if anything other then alert you if one half of the system fails ( mine is not wired so it doesn't even do that).
Joe
