I have a P2 with the same braking setup and when I had a similar issue, I just replaced everything with high quality ATE parts who make the Volvo braking components.
Cables, handbrake shoes, springs and new rear disc.
Overkill for some but the job was right and the job was done once and been fine for 3 years.
It holds on the flat with 1 click and at any angle with 3 clicks. My video of the job
is here.