IMO, "concurs" doesn't really mean much for a daily driven Mustang when it involves the single vs. dual master cylinder.

Lets put it this way, on cars my kids drive, conversion of the then working single m/c to dual was a priority. As a matter of fact, the single m/c on my 55 Lincoln was one of the first parts I changed out too. The thought of smacking the back of another car due to a blown rear wheel cylinder doesn't impress me much...

For a driver, originality for this particular issue means nothing to me. It is too cheap and easy to upgrade to a much safer dual m/c.

We simply used a 67 m/c with a 66 pushrod, plumbed the front and rear lines respectively. Worked great.

A 67 drum m/c will work great with your 4 wheel drums.


Back to the point, imo the problem is likely due to a broken m/c....

 

To all that responded to my brake questions,,THANK YOU VERY MUCH. The advice is well taken and I plan to start the conversion next week As the only running car in my garage this little stang can only rest a couple of days in a row then it must be freeway bound. For convenience and maintance the distributor was changed to "pointless" some time in the past so this will be one more up-grade for safetys sake.
THKS AGAIN JAMES92130

 

You will need to make one or two short lengths of brake line (depending on how you plumb) to connect the front and rear lines respectively, and you will need to double flare the ends.

The push rods are going to be hard to yank out of the m/c's, but a vice and some locking pliers will get the job done.

 

If you end up installing a new master, be sure to bench bleed it before installing it in the car.