Rear discs on these suck, I've done exactly the same as you numberous times and they still don't work. Left it with a garage after an MOT and told them that they just don't work and to check them over, told me I needed a new caliper, so I fitted a spare one, which still didn't work.
Next thing I know, it's passed its MOT and guess what, the handbrake still doesn't work lol. Even the footbrake has really low efficiency on them.
If you want a working handbrake on a Toyota, fit drums. Haven't touched the ones on my corolla in 9 years and they still lock the back wheels!