Rear brakes lock up, front factory disc 67 Barracuda

dano said

I believe in your case there should be two factory valves, a second one to the rear. Did you install that as well?

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Yes, there is a second valve on the line to the rear brakes on earlier cars. I'm not sure where the cutoff was. 68 cuda has it, but by 73 it was all in one. If you cannot get one, the fix is described above by dano.