First step to safety as MoparLeo stated is upgrading. I did the MC swap first. If you're running 13" tires than that must be converted over to 14" when doing a Disc brake swap. Took me a couple years to save up for complete rim, tire swap along with disc brake parts (found everything on this...
When I did mine, I followed the article I posted and did a direct MC swap. Have done it on all of my 64's and 65. Took about 4 hours with most of that time at the parts store finding the correct fittings.
Use the same rod. Just unbolt old and bolt on new master cylinder. no need to touch the rod. The stop plate and boot are on the inside side of the firewall.
Early Valiant Barracuda Club - dual master cylinder
"This Kelsey Hayes disk brake swap should work for any A-body with the 5x4 small bolt pattern wheel, EXCEPT those with 13” wheels".
Why does this swap not work for cars with 13" rims? Are they built differently?
Have you tried having someone push down on the pedal while you have 1 bleeder open? Make sure you close bleeder before they let up on pedal, do this for about 6 times and see if fluid eventually makes it to the back.
When i changed out my engine this spring I did the change over then. hardest part was finding the correct fittings at the local parts store. Much easier to remove distributing valve and working with it on the bench also.
sounds like the "ball" inside the proportional valve is stuck to one side due to a pressure imbalance between front and back brakes as was stated earlier.