If you are using the original B10 rear end & brakes, & I think you are, then it seems to me that the rear wheel cylinders have siezed.
Siezed to the backing plate that is.

The wheel cylinder is designed to move a little in a slot in the backing plate so as to centralise & to apply even pressure to both brake shoes.

You will probably need to strip the brake down to fix it properly, so undo the brake cables, & with a proper 10mm flare nut spanner, remove the brake tube. Next, remove the rubber boot that covers the external parts of the wheel cylinder, study the parts carefully & you will see how to remove the spring retainers & adjusting plates. Remove these & the wheel cylinder can be removed, assuming that you have also removed the brake shoes beforehand. Once these are out, the whole cylinder can be removed from the backing plate & you can clean all the parts with a wire buff if you are carefull. Then reassemble using a little anti sieze lubricant, & check for full & free movement.

While it's apart, you may as well remove the brake adjusting 'wedge bolt' It comes out on the brake side of the plate. Clean all parts & again, use a little anti sieze lubricant, then refit. Reassemble everything, adjust & bleed brakes properly. Make sure that the handbrake cable is slack when doing all this.

Once you have everything right, then [& only then] you should be able to get a proper adjustment on the handbrake. You may find that the brakes settle in a little after a drive round the block, so you may want to fine tune the adjustment after a day or two. Always adjust the wheel brakes first, & make sure that the handbrake cable has some slack when doing it, then do the cable adjustment last. Everything will be fine then.

Hopefully, this has been of some help.
If you don't have a workshop manual for your B10, but want one, drop me a line.

As a matter of interest, according to my parts book, left drive B10's have a floor mounted handbrake, the same as the KB10 does, while right drive B10's have an underdash brake handle. If handbrake cables can be sourced in a left drive country, then KB10 owners have a source for these parts.