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The prop valve is needed when you have disc up front and drums in the rear. The drum brakes self energize so you need a valve to "blow off" the pressure. Otherwise the rear drums will always lock up during a hard stop.

With discs on all four corners you don't really need a prop valve since everything is linear. But you can get into a problem if you change the weight distribution or drive in different weather conditions. Thanks Andy! The info I have which is gleaned from your B-Body Book shows the 2004 Viper Calipers as 5.480 inches of Piston area. The rear Cobra Calipers are approx. 2.25 according to my Calc. this should be close to 1/3 the braking force as the fronts. But I did not Calc. the rotor size so it's a "guess",, 13x1.250 fronts 11.7 X 1" rears,,, the book really helped me,,thanks!!
Best thing to do is to set up the brakes so you're starting off with the correct proportions front to rear. Typically you want the rears to do 1/3 of the braking. So you set the rear up to have 1/2 the brake force of the front brakes. That rule of thumb works for most nose heavy muscle cars.