You can pinch off all the rubber brake lines with some pinch off pliers or vise grips and see if the pedal is firm.
Make sure all air should be out of the system. I like to gravity bleed.
If firm, the problem is below the clamps. Then remove one at a time or only from/only rears and see if the problem is found.
If still soft with the hoses clamped off, then there are ways to isolate each components between, but if this is on your 79 then it can really only be the master or adjustments if available. You can use the plugs you used to bleed the master with it still on the car and see if it is still soft.
I have my doubts on a proportioning valve, all that does is control the pressure so that the rear brakes get a little more force. I think more of a lock up issue would happen.
Sometimes safe hard stops fix it all. Like something gets pushed into place.
Let us know.