Carlos, if you need a spring just go to your local hardware store. Pretty much any one will work.
It takes great time&skill to master using rear brake.
I'm now very cautious about using rear brake. Lack of proper technique and experience cost me high side on a street (thankfully only colar bone broken) . Now I'm doing 80% of braking with front only.
When I'm using rear brake (i think) I don't use it as braking device but to stabilize chassis on turn entry. I barely put pressure on lever.