If the car pulls to the right it means the opposite caliper is not working at all or very much! It is free rolling around the one that is slowing the car down.

Have you tried loosening the brake lines from the calipers to see if they are flowing? It is possible that the octopus block, that's in between all the calipers, has a partial blockage too!

The pistons and the bleeder passages from the chambers are probably rust plugged. The passages are not very large.

All depends on how long it's been since the system has been serviced.

It is recommended to service the fluid out every couple of years. I exchange the fluid out of the reservoir yearly or so with a baster syringe, even if I do not bleed the brakes all the way around exactly every two years. More like three + is my average. I have change out one caliper in all my years of owning the same half dozen vehicles all my life and that was about three years ago.

My theory is that replacing part of it with new will allow it to absorb moisture out of the remaining fluid since its connecting somewhere in the total mixture. Without the O2 of the water its hard to make things rust in a fairly well closed up system.

Everything works better when you have flow or in my life, I have learned to be more fluid!
Phil