Replenishing the reservoir does not introduce air into the system.
Almost every rear caliper I have bled needed to be removed from the carrier and held in various positions to allow the entrapped air to do what the bubbles want to do naturally......flow upwards.
Once you get the piston operating correctly you will need to ensure that the rod is adjusted to allow some free play and not block the return port. Otherwise your next ride could be shorter than planned.
Don't allow yourself to become frustrated. Remember, you have to be smarter than the machine.
Old and (not so) Dumb is correct. Having been through that before, I bled the whole system on my bench before installing it. Worked a treat.
I still have my doubts this is what's causing your issue, but smarter ppl than me are here to help. I just know I had the whole setup with my pedal and clevis rod functioning beautifully before I'd even installed the brake, fluid, and bled. If you stand there with the master cylinder in your hand, pressing the rod in, it should be smoothly backing out on it's own. Just seems like you must have some friction/interference going on with your rearsets install. It doesn't take much at all, a mm off will do it.
