I've had several Fords that have had problems such as you have described and the "cure" was to disassemble, clean the slides, chip off any rust, lube with high temp grease then reassemble. My '79 Mustang needed to have that done every 12-15 months. The most costly one was my current '06 F350 which cost a pair of calipers, rotors and pads. Now I 'fix' it yearly as well.

And that proportioning valve is probably not needed.

Also, check the MC for a resid check valve too