Out of frustration regarding bleeding the rear brakes, I've tried almost every combination of bleeding possibilities I can think of to get the rear brake to be as stiff as the front. I've drawn fluid through the line both directions. Bottom up and up down. With the lever bleed port level, I have carefully topped off the lever while very slowly moving the lever. With the lever bleed port elevated at a slight angle, I have carefully topped off the lever while very slowly moving the lever. Never seems to make a difference.
Therefore, I concluded that tiny bubbles must get trapped somewhere in the caliper.
Bleeding the rear caliper, I have tried to bleed with the caliper/complete bike elevated higher than the lever assembly, or lower than the lever assembly, tried moving the caliper in all sorts of angles, while gently tapping on the outside of the caliper.
Not matter the approach I used, the rear brake lever always seems to be softer than it should be.
Therefore, I would pump the lever a lot, with the caliper elevated as high up as I could get it on my bike stand, let it sit overnight, then bleed through the rear caliper. I went through this routine 4-5 times, and eventually I was able to get the lever to be stiff and pull the same as the front.
To date, I have presumed that I am missing something simple regarding my bleeding technique.
I was used to bleeding XT calipers and never had the issue I have had with a IIIPro rear caliper.
