Hay Zeke, I was and ASE certified Brake Specialist and a dual Light duty vehicle and heavy truck Master Mechanic for 30-years. In my world, I only worked on factory unmodified vehicles, so I didn't need to be a brake engineer.
I have had 15 early Mustangs and I have rebuilt lots of their master cylinders. I think they all have a residual valve inside of them. These photos are from my 1970 factory Ford Mustang shop manual. The combination of two of them is probably too much pressure for the springs to retract the shoes. Take that extra valve out.
All drum brake vehicles have a residual valve that holds some pressure in the system
This keeps the rubber cups pressed tightly against the cylinders' walls so they don't leak when your not applying pressure. It will hold pressure in the system for days or even weeks. This prevents the cups from leaking but also prevents moisture from being pulled back into the hydraulic system. Maybe the combination of two valves holds the shoes out while the brake fluid is hot but as the fluid cools it shrinks taking up less room in the cylinders and lines which allows to shoes to return. If you have a car not driven very often get in it every once in awhile and stomp on the brakes to reset that residual brake pressure.
As far as the line size depends on the wheel cylinder size. Early Fords used 1/4" lines where as Mustangs and most newer cars use 3/16". 3/16" is probably okay for early Fords but since the engineers used 1/4" on the Early Fords I would go with that. Where you might have a problem is if the Mustang master cylinder has 3/16" fittings and the wheel cylinders have 1/4" If that is the case you are going to need adaptors for the master cylinder. The F-800 trucks where I worked had very large wheel cylinders, two on each wheel if I remember correctly and they used 1/4" lines. The pressure will remain the same regardless of the line size but the 3/16" lines would restrict the flow to the large wheel cylinders which could cause a momentary delay in braking.
If you look at the photo that is a combo disc brake and drum master cylinder The disc brake is always the rear portion of a dual master cylinder. You will notice there is no residual brake valve on the rear port. That is because disk brake caliper pistons have no return springs they rely on the square seal in the cylinder twisting as the pistons moves outward and the seal un-twisting is what pulls the piston back .005, just enough that the front pads don't make contact A residual brake valve would hold the pads against the rotors all of the time and wear them out.
I'll go see if I can find my boxed '65 Mustang shop manual upstairs in the garage and see if they show an internal residual valve in the single master cylinder.
They show one but they are calling it an outlet check valve in the illustration and a residual valve in the description. There are no specifications in there for the pressure rating but if it is good enough for Mustang drum brakes, it will work with your drum brakes.
I'm curious since you didn't stay with the traditional Early Ford master cylinder why use a single Mustang master cylinder instead of one for a dual primary and secondary system, it's a lot safer. Besides having a wheel cylinder leak all your fluid out of a single system you can hit something and have it take out a steel brake line or even rip a hose off and then you have no brakes. Every once in awhile it happened at work. If they make a Mustang dual drum brake master cylinder with the same mounting and piston size I'd use it instead.
Others have said it but I'll repat it you need some air space in the master cylinder resevoir and you need just a little free play in the pedal rod, just a little, you don't wan *** falling out. Just enough to make sure the master cylinders' piston is coming all the way back against the snap ring, It it does come "all the way" back the piston cannot uncover the little port that allows the fluid to go from the cylinder back into the reservoir. After a few pumps of the brakes they will be locked up solid and you will not be able to move the vehicle without opening up a bleeder. I don't think this is what's happening to you since you drive it for along time and the pedal eventually gets hard but they are not completely locking up.
