Hey, I have a disc/disc combo valve, I think im gonna change the master and go with new rear calipers.that master cylinder is notorious for being bad rite outta the box and bleeding issues.due to the angle it attaches to a booster it's pretty steep as in jacking up the back to level out the fluid level in both halves and screw the cap back down.have you thought about replacing the prop valve with a disc/disc one?i switched to four wheel disk and used one and have zero rear brake lock up issues.i also installed an adjustable one early on and it's not needed with the disc/disc combo valve.