Agreed, but I don't think you need the check valves to delay brake application. That's more of a safety thing designed to help the inexperienced masses control their cars.
I suggest this: Go to the parts counter and ask to look at their brake catalog(s). Look up the master cylinder that came with front disc brakes like the ones you have; like from an applicable year mustang. Use the catalog to identify the stroke and bore of the master cylinder's front plunger. Now, do the same research for the rear brakes. Then find a master cylinder that fits your booster and has a combination of the proper front/rear bore and stroke. What this does is properly provides the right base pressure to the wheels based on the size of the calipers, wheel cylinders and master cylinders.
It sounds complicated, but i think you'll find that almost all master cylinders from disc/drum cars are remarkably similar across the board; GM, Ford, Mopar, even some imports. If you have to find one that is just "close" you're still probably OK, but err on the side of more piston bore on the rear plunger.
Now, once you find that M/C you can install it, along with an inline variable proportioning valve on the rear brake line. They're under $40 from companies like Wilwood. Adjust it to wide-open, then take the truck out and apply the brakes until they lock up. Its almost guaranteed that the rears will lock up first. Keep adjusting the knob to limit rear pressure until you get lockup evenly at all four wheels.