Each V-brake unit has a balance screw. You’ll find it at the bottom of the unit, usually a crosshead bolt but occasionally a small Allen key. The end of each bolt rests on the end of the brake-return spring, so that the spring is forced against the bolt when you squeeze the brake unit towards the rim.
Turning the balance screw alters the preload on the spring, pushing its starting point further around the unit for a stronger spring action and releasing it for a weaker spring action. The confusing part is remembering which way to turn the screws for the effect you need.
- Turning the balance screw clockwise (A) pushes it further into the unit, increasing the preload on the spring, making it springier and pulling the attached brake block away from the rim.
- Turning the balance screw anticlockwise (B) unscrews it from the unit, decreasing the preload, softening the spring and allowing the brake block to move nearer to the rim.
Since the two units are connected together by the cable across the top, adjusting one balance screw will affect both units: if one unit is pulled away from the rim, the other will be drawn towards it to compensate.
To adjust the balance screws, look first at each brake unit from face on – the front brake from directly in front of the bike, the rear brake from directly behind.
If the balance screws are badly adjusted, the units will point off to one side, rather than being parallel and vertical. There will be an uneven distance between brake blocks and rim, perhaps with one closer than the other, or even with one brake block dragging on the rim. To correct the problem, locate the balance screws. Start with the unit that’s closer to the rim, and wind the balance screw in (clockwise) a couple of turns. You’ll need to squeeze and release the brake lever every time you make a balance-screw adjustment to resettle the position of the spring.
Look again at the angle of the two units. You should find that the adjustment has both pulled the closer brake block away from the rim and pulled the other block closer.
One confusing thing about the balance screws is that turning the screw has a different effect at different points – sometimes a couple of turns seems to make no difference at all, sometimes a quarter-turn makes a radical change. You’ll have to experiment, adjusting the balance screws a quarter-turn at a time to find the central position.
Modulation is just a fancy word for ’how much the cable travels when you pull the lever’. Adjusting the modulation means changing the distance between the point that the cable attaches to the lever blade and the pivot that the lever turns around. Increasing this distance means more power, but it also means more lever travel. Some levers have an adjustment for this, usually a thumb screw on the front of the lever. In the example (see picture on right), the red thumb nut situated on the front of the lever adjusts the position of the cable nest. Turning the thumbscrew clockwise moves the nest further away from the pivot of the brake lever, so that more cable is pulled through the lever when you move the lever blade. Turning the thumbscrew anticlockwise moves the nest nearer to the pivot of the brake lever, so that less cable is pulled through when you move the lever blade. Adjust the lever modulation so that it gives you a comfortable amount of lever swing. This will depend on the size of your hands.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES