Every model of brake works a little differently, making it impossible to cover each and every one comprehensively here. Always keep the instructions which came with your own particular brake and refer to it for detailed instructions relevant to your brake.
The basic principle is the same for all hydraulic brakes, though: the pads need to sit close enough to the rotors to be able to bite swiftly and firmly onto the rotor’s surface when you brake, but they need enough clearance to allow the rotors to pass freely between the pads without slowing the wheel down unnecessarily.
It’s fine for rotors to rub a little on the pads. Disc brake pads are much harder than rim brake pads, so a little bit of contact isn’t going to slow you down and light ’scuffing’ at one or two points on the rotor’s rotation isn’t a problem. However, a badly bent rotor or poorly adjusted calliper will cause the pads to rub on the rotor permanently, which will slow you down and cause unnecessary heat build-up in the fluid as well as pad wear. Slightly bent rotors can be straightened by hand but warped rotors must be replaced.
The most common design for mountain bike hydraulic disc brakes uses two pistons, one on each side of the rotor. This is more than powerful enough for the majority of riders, but if you decide you want to go faster and need to look at brakes which are up to the task, then you could look at the more powerful variation which uses a pair of pistons on each side of the rotor (four in total). These are designed to fine-tune the action of the brake by making two of the four pistons slightly smaller; the pair of smaller pistons moves first, followed by the larger, pushing a larger brake pad. The greater surface area of the pad is less forgiving of maladjustment but helps dissipate heat effectively on long descents or under hard braking as well as giving more power. If this four-pot system is still not enough, then you could take the final step and fit a six-pot system; but if that’s the case you’re probably well beyond any advice this book can offer!
Brake pads can be removed easily for inspection or replacement
Adjusting pad clearance
Whilst older and more basic open hydraulic systems don’t allow you to adjust the pad placement in the same way as a closed system does, many now come with a bite point adjustment in some form or another. The simplest of these are the tool-free versions found on Hope’s Tech Levers or Shimano XT and Saint. Look into the gap between pad and rotor whilst adjusting the dial to work out which way the pads move; then turn the adjusting dial so that the pads hit the rotor when the lever is approximately halfway to the bar. This gives you enough lever travel for precise control over your speed but avoids any risk of trapping your fingers between bar and lever when braking in a hurry. Another useful adjustment is the lever reach. On a few levers this is also tool-free but the majority require you to use a tiny, 2.5mm or 2mm Allen key to turn an awkwardly placed screw that’s usually located on the inside of the lever blade. Don’t be tempted to try and hurry things along by using a long, ball-ended Allen key for this unless it’s a proper T-bar version – a basic one will round off quickly as the short end of a regular Allen key doesn’t give you enough leverage to turn the screw effectively.
To figure out where you should set the lever reach, sit on the saddle and place your fingers on the bars as if you were about to brake. If you have to stretch at all or find yourself rotating your hand around the bar in order to get your fingertips on the lever, then you need to move them inwards. Ideally your fingers should rest with the leverblade sitting between your first and second knuckles.
It’s important to remember that the majority of disc brake systems are self-adjusting and so you’ll need to pump the brakes frequently when making adjustments, to let them settle into place. This applies if you’ve fitted new pads, bled the system or are adjusting the bite point, as well as any other maintenance task. If you pump the brake lever and the lever comes all the way back to the bar before the pads bite, you’re short of oil volume and probably also have air trapped in the system. Follow the instructions for bleeding your brakes on page 124–5. On the other hand, if the brake pads bind on the rotor no matter now many times you pump the lever, you may have too much fluid in the system. With care, you can drain a little of this excess away. Fit a length of clear hose to the brake calliper’s bleed nipple to prevent air being sucked in, then open the bleed nipple a quarter-turn with the correct spanner. Squeeze the brake lever gently so that 3–4mm of oil creeps up the hose – no more. Close the bleed nipple, remove the hose and test operation, before repeating if necessary.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES