1000 miles isn’t a lot......I’ld ‘cook’ them a bit...anyway....what have you got to lose....if everything else checks out....aren’t many options.
I initially did not do the fronts, but I have since. In my 30 years of turning wrenches I’ve never had air move from one wheel to another when working on one thing at a time.you keep saying you've bled the brakes with no air bubbles, have you been doing the front also? both cylinders at each wheel?
I'm back to my original thought that the fronts may not be doing any of the work