[JLMounce]

I see two basic issues here based on what you've provided for information.

First, I agree with scarebird that the master cylinder is too small. You have 7.065 sq/in of piston area in the front. Those things need a lot of volume.

The second issue is that depending on the volume ratios in your mater cylinder (apologies, I don't know much about the mopar stuff) you may have an issue between the static bias of the calipers. Most GM and GM modeled mater cylinders like wilwood tandem will have a 2:1 volume ratio between circuits.

Your rear calipers have a piston area of 3.14 sq/in. which is 2.25 times smaller than the front. It appears you may be compensating for this issue by running the primary circuit to the front brakes and the secondary circuit to the rear brakes. If that's the case and the volume ratio of your master is similar to the GM design, you're additionally starving the front calipers of volume, which is effecting clamping power and stopping ability.

Again, all of this assumes the mopar master is setup similar to a GM. If not, I'm basically wrong on all accounts here.

But the way I would fix this would be to get a GM style master in a 15/16" size with a 2:1 volume ratio. Then, I'd either upsize the rear caliper slightly, or downsize the front caliper slightly. If you can get a Wilwood dual piston D52 caliper in the front in the 6.28 sq/in model, you'll have a near perfect static bias between the calipers. You'd then be able to better dial in the rear brakes with your adjustable proportioning valve.