have u checked and/or serviced the handbrake mechanism in the caliper is still working smoothly? handbrake cable lubricated & sliding smoothly?
rear brake discs are very inefficient by design (required pressure per brake force) compared to a self-energizing drum brake.
but what I found recently was the bias proportioning compensator valve fitted at the rear trailing arm on models with/without discs was mainly to limit peak pressure to rear drums to stop em locking.
so on models with discs (which actually need more pressure to work) they left the valve in so the discs tend to not work very well.
what I've done this year was fit an adjustable bias valve and removed the old poor proportioning valve and whilst bedding the brakes or going on trackdays, the rear discs are able to lock if the valve was set close to 50/50% bias.
in terms of the handbrake lever, yea they're pretty spongy poor n not enough leverage. perhaps replace or mod the handbrake with a shorter throw? grippier low temp rear pads, grooved rear discs to bite the pads?
combine both bias adjuster & harder handbrake lever & trail brake?
