hiya
i know on mine it was on a sort of ratchet system as the pads wore down on the rear, it adjusted its self.
i had all the breaks done recently, that resets it to its lowest setting if you get what i mean.
so maybe your rear disks and pads are due for replacement, also check your brake fluid.
i pumped the handbreak up and down a good few times also.
sorry if i have not been much help
i know on mine it was on a sort of ratchet system as the pads wore down on the rear, it adjusted its self.
i had all the breaks done recently, that resets it to its lowest setting if you get what i mean.
so maybe your rear disks and pads are due for replacement, also check your brake fluid.
i pumped the handbreak up and down a good few times also.
sorry if i have not been much help