Thanks to all that have posted - I am learning a lot about this. Unfortunately the wreckers around here don't seem to have much of the older stuff in their yards anymore, so I am having trouble finding anything. Probably have to go with a rebuilt unit from a later vehicle. Just want to check something first. It is my understanding that the master cylinders are all pretty much the same in the way they function and the proportioning valve is what does the work to control pressure distribution to avoid lock up. Will switching proportioning valves when I go to discs be a problem?