This is going to be a long post so hang on!!This has become a baffling question because of some misunderstanding of the part functions and imporperly named parts. The function of a proportining valve is to 1: allow the rear brake to apply first,which reduces wear on the frt brakes. Then as pedal force increases so do the line pressures, so 2:the proportioning valve steps in again to limit the pressure been applied the the drum whl.cylinders.The dics brake line pressures greatly exceed the drum brake line pressures so 3:it allows the pressure applied to the disc to continue increase as needed to bring the vehicle to a stop. This function"does" help keep the rear brakes from locking-up but, it's design intention was not anti-lock.This function is more of a safety feature to keep the rear wheel cylinder from "blowing- out"or leaking. I do not intend to make any one mad but in most cases the valve that is beening sold and used on the rear brakes as a proportining valve is actually only a "Pressure Regulator Valve". I am not saying the does not work or help but a brake system which incorporates a "factory" or "true"proportioning valve will function better and have more "brake life". To clearify the use of daul- master cylinders, I believe there is two intentions here--1: safety again as it actually gives you two braking sytems. 2: drum and disc are two differnt sytems and have different valve requirements--more on this later. The drum brakes need a residual valve because they are return to rest position by springs.Rotational force repositions the disc Pads so it does not need a residual valve when the M/C is mounted above or higher than the whl.cylinders & disc. pistons. The factory chose to incorporate the residual valve in to the master cylinder. If you want,I will explain "Residaul valves",thier function and application in another post---I have some work I have to go do right now--paul
