I think your calipers are just knacked dude. Mine did exactly the same.. no amount of fiddling with it sorted it out.
The culprit seems to be the auto-adjustment feature on the handbrake mechanism... with time (and the beast must be @ 13 years old now..) the thing corrodes up or something and only a complete rebuild will clean it up.
I would not arse about with it.. buy new (recycled) calipers from www.speedways.co.uk
These fella's take your old calipers and send you someone else's old ones that have been professionally refurbished using new parts. They look and act like new parts. About £90 a caliper, which sounds dear until you work out the cost of the various overhaul kits, new pins and other bits and pieces you'll need. Just paint the new ones red and slap them on.. job done.
You should also look at the brake bias adjustment spring. The Brake Bias adjuster is supposed to direct more brake force rearwards when you have a heavy load in the boot... But it has to be adjusted right.
If the spring rusts "extended" (as they do) or the adjustment slips towards the front, then the rear brakes get hardly any action, so they seize up more quickly.
The fella lives "under the passenger seat" (but access it from under the car). It's like a little cylinder, with an arm under a rubber boot and a spring attached to it. Move the rubber boot, loosen the locknut... add 10kg of weight on the arm (5 bags of sugar... you can guestimate that) and then tighten it all up.
I tried running max rearwards bias (about 20kg of weight!) to force the rear calipers to do more work but once they're knacked, they're knacked.
Your brake discs should be as clean as the fronts.. bet they aren't.