A cable operated rim brake has three elements. The lever, the cable and the brake itself. Separating the elements helps decide which element needs attention, and where your brake problems lie.
Dismount the cable to isolate the various elements, and help diagnose your brake problems.
Once the cable is released you can inspect the operation of the brake and check for further brake problems.
Push The Blocks Against The Rim.
Does all the block hit the rim? Does the block hit the rim flat?
Release The Blocks
Does the return spring – or springs – push both blocks sharply away from the rim? If the brake has two arms – V brakes, or cantilevers – do the springs push equally hard?
Pivots too loose?
Brake problems can stem from loose parts. push each brake arm forwards and backwards to feel for looseness in the pivots.
Inspect the cables. Are the inner-cables shiny? Are the outer-cables free from cracks or splits? Does the inner-cable slide freely inside the inner-cables?