So there's two types of valves used in automotive brake systems.  There's a proportioning valve, and there's a shuttle valve.  Some vehicles have both valves in a common body making it a combination valve.  The proportioning valve adjusts pressure to the rear brakes to prevent premature rear brake lockup under hard braking.  The shuttle valve is the other half of the safety system provided by a dual reservoir master cylinder.  If there's a leak in either the front or rear circuit the shuttle valve gets pushed over to block the leaking circuit's feed passage.  This prevents the leak from completely draining the system of fluid, and allows whichever circuit hasn't failed to continue to stop the vehicle. 

From the factory in '66 the Mustang has a single reservoir master cylinder, and thus no shuttle valve as there were no separate front and rear circuits.  There was just a brass distribution block, basically a four port tee allowing the line to the two front wheels and the line to the rear of the car to receive pressurized fluid from the single reservoir master cylinder.  Those distribution blocks should effectively never go bad.  I don't think they even have seals to leak. 

If you had disc brakes from the factory they installed a proportioning valve in the line to the rear brakes.  Here's some info on the factory valve, which does appear to be rebuildable provided the bore is not pitted or damaged:

http://www.stangerssite.com/brakevalve.html

My advice, since you have a dual reservoir master now would be to install a later or aftermarket distribution block with a shuttle valve in it.  Without the shuttle valve the dual reservoir master provides no real benefit because the dual circuits coming out are just tied back together in the factory distribution block. 

Then there's the question of proportioning.  If you have stock brakes front and rear and stock size wheels and tires (or close to stock) you could probably just use a stock proportioning valve.  If things have been modified or swapped I'd suggest getting an aftermarket adjustable valve so you can properly set the bias.  This is the one I run on my '89 GT with '94-'04 Cobra brakes front and rear:

https://www.summitracing.com/parts/sum-g3905/overview/
 

Just added picture !!! Hooray !!

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I edited my original thread with picture it shows on my desk top view but not my cell view  ??