I've said it once, I'll say it a bazillion times.
DO IT ONE STEP AT A TIME.
Have you tried your stock master after the disc conversion? Many folks, myself included, have had good results with a (new) stock master and prop valve. I suspect that many folks who've had trouble after the disc conversion had the same problems before -- i.e. a leaky or worn master cylinder.
IF, after doing the disc conversion, you have troubles -- and the master cylinder is new or known good -- then you can put in another style master. Be sure to match it to your booster, i.e. vacuum boost or hydroboost.
THEN if you have issues with the front and rear locking up at different times, you could swap out the prop valve. Yes, if you put an adjustable one in, you'll need to replace the stock one with a tee. to pass the one line from the master cylinder to the two hardlines to the front brakes. Then you just couple the other line from the master for the rear brakes to your rear hardline and off you go. You will become intimately familiar with the brass plumbing parts at your local good auto parts house.
BTW, I've been through all of this on my other truck ... took me months to get the dang thing together, and in retrospect, I'd prolly have had the same luck with new stock parts.
Point being, don't start throwing parts at a problem you may or may not have, and don't assume that parts for some other application are automatically better than the originals.
-- A
