I think you need to figure out f this is a hydraulic problem, or if it is a mechanical problem.
Lets attack the hydraulics first;
Crank the rear adjuster hard so you cannot even spin the wheels; now go step on the pedal. If now high and hard you have no hydraulic problem.
But if still excessive travel; remove the calipers and C-clamp the pistons into the bottom of the calipers then pedal-test. If now high and hard, you have no hydraulic problem.
But if still have excessive travel,or soft/spongy pedal, that would be air, or bad hoses.
If the pedal falls with continuous and constant foot pressure; fluid is leaking somewhere; it could be external, or if you cannot find an external leak, then the M/C is leaking internally and since it is new, I would warranty it.
There should NOT be a residual valve in the line going to the calipers.
If your rear wcs are new, then a residual valve to the rear is optional, as modern wcs have expander springs inside them that do much the same thing. If they are in there, I just leave them be.
With factory Mopar brake systems, the frontmost reservoir should be plumbed to the rear brakes.
Your car, should probably have a Combination Valve. This item has 3 functions; 1) as a distribution block, 2) a rear proportioning valve, and 3) a safety switch to turn on your parking brake lite, if either side of the system fails. If you don't have a lite then you can use something else, but you still need to Proportion the rear brakes....... unless maybe if you have big-N-litte tires.
As to the mechanical side;
If it should happen that the above tests prove that the hydraulic system is functioning fine, then you have a mechanical problem.
With the rear shoes still jammed up, install just one caliper and pedal test again.
If excessive travel again; go watch the caliper as a helper works the pedal. Something is probably flexing. Usually I find it in the pad alignment. The pad faces have to be parallel to the steel backing, and sometimes the anti-rattle material gets crossed up. Do not use any RTV as a silencer; if you did then remove it; it just acts like spring. The second place I find flex is the interface between the steel backers and whatever anchors them from rotating.It could be the caliper brackets or the slider-pins depending on which system you have. Or it could be rotor run-out pushing the pistons back into the bores while driving. Then you gotta pump them back out to get brakes. I have never seen a caliper flex.
So after you find and fix the flex, do the same on the other side, and then pedal test .
If the pedal is now hi and hard, relax the rear adjusters, and road test.
But if still wonky, remove the rear drums and shoes. C-clamp the wc pistons into the bodies and pedal-test. This is the last test so the pedal better be high and hard!
If it is, then lay a shoe into a drum and see if it rocks. If it does check all the shoes in both drums. If they all rock, take all the shoes and both drums down to the brake shop and have them ground to fit eachother. Reinstall, pedal test, and your problems should be over.
Why is this important? Because #1 if the shoes rock, they are only touching in the one spot., and #2, then you get poor rear brakes. This causes you to step on the brakes harder, which may cause the rear drums to go momentarily out of round, and they act like a big spring. #3,The shoes then become glazed in the little patch that was working so hard.That glazed area becomes smooth and hard, and offers very little braking action,and it will have to be either burned off with miles or ground off on a machine, so it can work again.
Happy hunting
