Rear brakes still locking up

Sorry Op
I see we've been down that road already.
Really, there are only three reasons that the rears will lock up before the front;
1) bad design, and 2) the fronts are not working properly, or 3) there is air inside the M/C between the front and the rear pistons.

as to 1) Well it's a factory Mopar so it ain't a poor design,lol.
as to 2) The rears are locking when braking firmly so we know they're working, right? So,
I gotta wonder if you put the P-valve in the right line.
And if you did install it correctly, then I'm going with the fronts ain't working, or not working right .So you gotta jack it up all 4 wheels off the ground. Then have a helper apply the brakes until the rears are dragging heavily, then go check what's happening at the front.
as to 3) If you had this situation, you'd have a lotta pedal travel, and it wouldn't get hard until she hit bottom, and it would not feel normal.

Now just in case you don't know;
as to the two lines that come out of the m/c; the frontmost line, nearest the rad goes to the rear brakes. It is normally operated by hydraulic pressure. But when the front-brake system leaks or has air in it, then the rearmost piston travels forward until it hits the frontmost piston and operates the rear brake system mechanically. Because the m/c is normally uphill at the front end, air in that inter-piston chamber gets trapped in there. When that happens, there is a gusher in the m/c when you release the pedal as the compressed air slams the fluid out. There are only two ways to get that air outta there; 1) remove the m/c and bench bleed it, or 2) with the brakes gently applied, loosen the frontmost brakeline on the m/c and let it out. It shouldn't take more than two pumps to get it all out, and you have to close the line before the pedal goes back up! Remember to protect your paint cuz most brake fluids eat paint.
That's all I got man, hope something works for you.