It's been a while.
I took the arm off, and realised it is slightly adjustable. One of the mounting holes are not completely round, but allow a bit of adjustment. I made the hole longer, making it more adjustable. This way, I was able to tighten the spring more, and got more brake power to the rear. Braking felt a lot better, but I still want to tighten it a bit more.
After slight modification: 2016-11-27 18.23.53.jpg
Original bracket : 2016-11-27 18.13.25.jpg