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Troubleshooting : V- and cantilever brakes

Symptom Cause Solution
Brakes squeak Brake blocks set flat to the rim or with the back of the block touching first “Toe-in” brakes so that the fronts of the brake blocks touch the rims first
Rims dirty or contaminated Clean rims with bike wash and nylon scouring pad
Brakes don’t stop the bike or don’t stop the bike quickly enough Brake blocks are set too far from the rim Set brake blocks closer
Surfaces of brake blocks are contaminated or have picked up debris Remove wheel, pick debris out of blocks with sharp knife, roughen surface with wet-and-dry sandpaper, replace wheel
Rims are contaminated or dirty Clean rims with degreaser
Brake blocks are worn out Check wear on brake blocks – replace if necessary
V-brakes – brake arms too close together, head of noodle jams on cable clamp when braking Reset brake units to vertical, reorder brake block washers appropriately
Cantilever brakes – link wires make unit angles too wide or too narrow Reset straddle or link wire angle, readjust brakes appropriately
Brakes pull on, but they don’t spring back from the rim when you release the brake lever Dirty or frayed brake cables, kinked brake outer casing Clean and oil brake cable or replace it
Brake blocks are set too low and have worn so that a lip of brake block gets trapped under the rim Remove wheel, cut off offending lip with a sharp knife, replace wheel, reset brake block position
V-brake noodle is full of grit or squashed Flush out noodle with light spray of oil or replace
Brake blocks wear very quickly Rim surface worn Replace wheel or rim
Brake levers take excessive effort to pull, brakes don’t release smoothly Brake pivots worn, corroded or dirty Remove brake units, clean and lubricate pivots

Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES