V-brake units get very tired if you use them hard. Every time you pull and release the brakes, the units rotate around the pivots, bending then releasing the spring. The pivots and springs won’t last forever, particularly if you ride in muddy or dusty conditions. Simple, single-pivot types (like the Avid units in the pictures, or the Shimano Deore types) tend to last longer than those with a complex multipivot arrangement.
All V-brake units are made to fit the same size and shape of brake pivot, so you can swap between makes and models without running into compatibility problems. New brake units are supplied with a set of new brake blocks. Once you take the price of these into consideration, a new set of brake units is a good-value upgrade.
Servicing V-brake units
Old brake units that are a bit tired can be revived with servicing. You’ll need to release the brakes, then disconnect the cable clamp bolt. Undo and remove the fixing bolts at the bottom of each unit, then pull the brake units off the brake pivots. You may need to wiggle and pull at the same time, especially if the brake pivots have become corroded. Use a small brush (old toothbrushes are ideal) and degreaser to scrub all the dirt out from the gaps between the moving parts. Hold the unit still and wiggle the spring – if there’s dirt in there, flush it out. You may find that you can pull the spring off the back of the unit. This makes it easier to clean behind, but remember to note the position and orientation of any washers or spacers. It’s best to work on one brake unit at a time; that way, you always have the other unit for reference if you get confused when reassembling the parts. Rinse the unit to get rid of residue from the degreaser, and oil the gaps between the moving parts. Move the spring against the unit to work the oil into the gaps, then wipe off the excess. Remove the cable clamp bolt and clean off any trapped dirt under the bolt head or the washer. Oil the threads of the cable clamp bolt and replace it. Clean the brake pivots carefully, removing any corrosion with wet-and-dry paper. Oil the pivots, then refit the brake units as above.
“It’s best to work on one brake unit at a time; that way, you always have the other unit for reference if you get confused”
Step 1 : Undo and remove the Allen key bolt that holds the units onto the brake pivots. It may be stiff to turn because Loctite is often used on these bolts to stop them from rattling free. The bolt heads can be quite shallow, so scrape them clean with a screwdriver before you try turning them with the Allen key – you risk rounding off the bolts.
Step 2 : Pull the old units off the brake pivots. Clean the brake pivots (A) carefully, removing all grease and dirt. If the surface is corroded, use wet-and-dry sandpaper to carefully smooth it. Put a drop of oil onto each brake pivot and spread it over the surface. This helps the brakes return smoothly after you’ve squeezed them against the rim.
Step 3 : Slide the new units onto the brake pivots. Line up the stub of spring on the back of each unit with the hole beside the brake pivot. You may have to undo the Allen key bolt on the back of the brake blocks and twist the blocks out of the way to line up the spring properly. If you have three holes for the spring, use the middle one. Refit the brake fixing bolt and tighten firmly.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES