The proportioning valve is built into the master cylinder. If there truely wasn't a prop valve, then how could it have a split point at 20 kg/cm2 and a reducing ratio of 0.4?

If your rears are dragging, it means one of several things... or maybe a combination.
-your rear caliper has seized. I really haven't seen this before, even on rears with 230k+ on them.
-your rears need to be bled. Possible, but you'd probably be leaking brake fluid, which would be noticable.
-ebrake is too tight. Look at the rear assembly and find the ebrake cable end. There's a rotary spring holding the arm back. Make sure that arm is all the way back against it's stopper.
-Sliding bars are stuck. I took mine apart and they were super nasty. Definitely doesn't help. Simply disassemble, clean, and regrease.
-The pad clips need to be greased. This fixed my persistent pad dragging problems. Just liberally apply grease to the clips.

So, to cover all the bases, get the car in the air and get the tires off. Unbolt the caliper and regrease the sliding pins. Pop the pads out and grease the clips and ears thoroughly. Reassemble and bleed. Check the ebrake cable where it attaches to the caliper. The good part is, you can put the car in neutral while it's in the air, and spin the brakes while listening for dragging.

Or if you're made of money or you don't want to fix it again for the next 100k miles, just buy 2 new rear calipers. You'll still have to bleed the lines, grease the pins and clips, and inspect the ebrake.