Did you bench bleed both reservoirs when you replaced the MC's. The reason I ask is the rear reservoir took much longer to bleed than the front.
Also...my proportioning valve was gummed up and jammed on a 79 I had and gave me the same issue, so it may be worth looking at cleaning or replacing (it was cheap enough for me in the 90's to just replace)
First, bench bleeding the MC is a must. Second, did you use a new rod or your old rod? Could be the push rod is the wrong size/length and the piston in the MC is not traveling far enough to engage the rear... You can also try gravity bleeding by opening up the rear bleed screw and letting gravity push the fluid to the rear. I had the same issue and it turned out that I did not get all the air during the initial bench bleeding. I was able to bench bleed again with the MC on the firewall and that did the trick for me.
Pressure differential valve in the combination valve is off center. Attach a test light to the connection on the valve, if it lights up the valve is off center. To center it, open a bleeder on the a front caliper and have an assistant slowly push on the brake pedal until the light goes out. Close the bleeder on the front. The valve is centered and you should be alble to bleed the rear brakes.
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