Re: Rear brake calipers... :D
I know this one... been through it all myself over the last month or so. I was advised at the last MOT that the pipes into and out of the regulator valve were getting a bit rough so replaced them all. I found that the valve 'airlocked' so took several attempts at bleeding. Basically I used a second person to stamp several times on the brake pedal, then hold it down to bleed the calipers forcing the air out of the valve. Did that a few times, drive around a bit then do it again and certainly noticed a difference.
On the brake testing machine the rear brakes were giving enough force but the drivers side was not good enough on the handbrake. A new cable did the job on that side as it had gathered damp inside and became sticky - it got in where the cable sits on the caliper. Made sure I lubricated the cable well and it now seems fine, also smeared grease to prevent damp getting in again.
Hope this helps!