You don't really NEED a proportioning valve, I reused my drum/drum d-block and the car performs and stops great. I have a disc/drum manual setup. I used a disc/drum '74 master cylinder and put a '71 rod in it for correct pedal height.
All drum brake cars have residual pressure valves, usually around 10 or 11 psi, that way pressure is maintained on the wheel cylinders so that the seals don't leak. Just high enough pressure that the shoe return springs can overcome it. If used with discs a drum master can cause the pads to drag and wear out quicker, damage the rotor from heat since calipers don't have return springs to pull the pads away. Disc outlet ports on aftermarket MCs sometimes have about a 3 psi residual pressure valve to keep the pads close to the rotor and, like you said, to keep fluid from draining back when the MC is lower than the calipers. A lot OEM disc MCs didn't have these valves, though.
I have heard this before (years ago, actually), but I wonder, because I have known a couple of people who used a drum/drum master cylinder with front discs on their Mavericks, and there was no excessive drag on the disc brakes (spun freely by hand)...
I have the OEM 72 drum/drum master cylinder, brake lines, and distribution block on my Sprint with disc set up and have no problems.
Yup, people sometimes get away with it. If it wasn't supposedly needed to have different MCs you know Ford wouldn't have went to the expense of using them, being how OEMs are such cheap a$$es. In my own car with front discs things didn't work right until I added a rear prop valve, some guys seem to not need them. I can't say if the drum/drum MC would have worked or not 'cause I installed the correct MC when I did the swap.
