I put new calipers on the 120 few weeks ago, like you only one piston actually seizing and whilst the brakes were superb before the bite now with new calipers is amazing, hope the rebuild goes well.
I recall playing around with the rear proportioning valve on my KZJ70, when i first got it too prone to locking the fronts, adjusted slightly too far which led to premature locking of the rears, not nice, but eventually found the sweet spot.