Reduced braking force in V brakes I have a foldie which has always had bad/weak rear brakes, which keep getting worse. To preempt the response that they are expected to be weaker than front brake, this isnt a question about weight transfer, or traction being lower in the back, or the rear wheel locking up sooner. Right now, if i were to brake the rear while moving, I barely feel any braking at all. I have tried to adjust it to be angled correctly and hitting the rim correctly but it seems to make the issue worse. 

I recently noticed that both the rim surfaces are a bit curved (the centre is about 1 mm thiner/deeper than the top/bottom edges (if that makes sense). One side is worse than the other, which made me feel it was a manufacturing defect, but now i see that the other side has a similar (but less pronounced) curve, which makes me feel it is just a wear thing. 

I also notice that the rear brake pads are very smooth to the touch. They still have the grooves vertically, which indicates that they arent worn out yet? Should i try to sand it down a bit to try and improve the braking? 

Is there anything else I can do? Is the rim wearing down normally or is that the cause of the braking issue? (I have been sorta compulsively trying to adjust the back brake nearly as long as i can remember, cant get them working as I would like.)

The bike is ~2 years old, ~1300km riden. Sorry for the rambly post.

Edit: The rim is curved, but the wear indicators are still quite pronounced. Does this mean the rim has bent rather than worn out? The brake pads are quite worn out as well, one of the edges of one of the pads has reached the wear indicator, so I have ordered a new pair. Removing, servicing, and reseating the pads seems to have given me some braking power for now. Remains to be seen if it lasts long. I could find a couple of small metal pieces(like 2-3 sub mm size) on the pad, but not enough to explain the wear. I wasn't able to hear them when I brake either so might not have been a major factor.
A 1mm curve on the brake track suggests that the rims are worn and you need new wheels; on the other hand, it's strange that they'd be so worn after only 1300km. I'm not sure how much that will affect your braking performance but the rims weaken as they're worn away by the brakes (of course) and will eventually fail if not replaced.