Proportioning valve for rear discs
IMO Leave the rear alone. Rear wheel disc conversions gain you little in stopping power and if you check the forums on rear disc conversions, you will find common issues like low soft pedals etc...
Upgrade the quality of the brake parts you have for better performance.
It is like upgrading from bias ply to Radial tires.
Huh? Bias ply tires are hot garbage for street use. Radials, even the all-season hockey pucks people like to use on these cars for the "look" are a significant upgrade over the OE bias ply tires in pretty much any metric worth considering.
As far as rear disk upgrades, they can make a significant improvement. Mopar muscle did a rear disk conversion on a '73 Dart Sport and checked the stopping distances between the rear drums and rear disks from 60-0. Their result was that from 60 mph factory disks up front and factory drums in the back the car took 133 feet, 6 inches to stop. After the rear disk conversion, the stoping distance improved to 122 feet 4 inches. It wasn't a super scientific way to test it, but it was better than most of the documentation out there. And the car involved was set up the same way as a lot of these cars, much larger rear wheels than fronts.
The online article is a total mess now, probably something with being converted over or moved to Hot Rod when MM was bought out. The final distance used to be a caption on the second to last picture, but I don't see the captions popping up anymore. The hardcopy article is easier to follow, but that's the way it goes.
Rear Disc Brakes - All Bound Up - Mopar Muscle Magazine
A low soft pedal is an indication of an improper install, or mismatched parts. Like adding anything aftermarket to these (or any) cars, you can't just assume that because someone sells it and it costs a lot that it's also the right component for the car.
As for the prop valve, you do need one. The factory systems that don't use one were all sized taking into account the swept friction areas, piston bores, weight distribution and even tire sizes used front and rear. Since rear disk brakes for these cars are all aftermarket items that weren't originally designed for this application, I can pretty much guarantee that running without a prop valve will not give you balanced braking. There are far too many variables on how people set these cars up to just run a straight distribution block and expect that to work. That is not to say a factory disk/drum prop valve will give balanced braking either, an adjustable valve of some kind is pretty much necessary for most set ups. Unless you're going to bust out the physics book and start doing all the calculations to size the pistons, swept areas, CG, etc.
