I haven't had dealings with a Ka, but i assume it's disc front drum rear.

First thing i'd do is check that all caliper sliders are free and lubricated, and that caliper pistons are free moving, that applies if it has rear discs too.

Then i'd remove the rear brake drums and check for free slave cylinders (and leaks), and assuming the rear shoes are good check the automatic adjusters are working correctly, if necessary manually adjust them a few clicks at a time before offering the drums back up.

Not an uncommon problem with the Ka. It is possible to improve the brake efficiency by taking the rear drums off and tweaking the adjusters up a couple of clicks.