see if I am reading this right, you unhooked the forward (closest to radiator) line in the MC (for rear brakes) & hooked the short bleeder line to it (the MC port) & run it back into the MC bowl (just like you are bench bleeding (that half) of the MC) & the system felt "normal" as far as the front pads engageing? If all good on that & the other half (for rear brakes) of the MC is bled out then there has to be a blockage from that half rearward as you said. if we're thinking the prop valve is fubared I'd almost wanna go back to your parts house & get a short length of prefabbed 3/16 brake line with the male inverted flare ends and a double female inverted flare union and a 3/8-24 male inverted flare plug to cap the hole in the prop valve that the rear line was in that you took out, to bypass it COMPLETELY & see if the prob is there or further downstream. it'd be some work but that is my 2 cents worth at the moment