Hydraulic brakes are much more powerful than cable operated ones. The principle is simple: pulling the lever towards the bars activates the master cylinder, pushing fluid down a narrow tube to the calliper, acitvating the slave cylinder which pushes the pistons outwards. The pistons push against the brake pads, moving them inwards until they contact and grip the rotor attached to your wheel. The brake fluid is incompressible – any movement of the lever is transferred directly to the brake pads via the two cylinders and the pistons – and a hydraulic brake can be very powerful indeed. The idea of dealing with hydraulic fluid can be daunting, but as long as you’re calm and careful, it’s not difficult at all.
There are two types of fluid that are commonly used in mountain bike disc brakes and you MUST use the right one for your brake – using the wrong one will damage seals throughout the system irreparably. DOT fluid has a higher boiling point and expands less at high temperatures, whilst mineral oil is easier to work with, less corrosive and less environmentally damaging. There isn’t a significant performance difference between the two when used in mountain bike brakes, just don’t confuse them. Both types will eventually absorb moisture from the air and become less effective, so buy small amounts and keep them sealed, disposing of leftover half-bottles responsibly if they’re been hanging around for a while. Wear rubber gloves to protect yourself when using both types of fluid, as they are damaging to the skin.
One of the indicators of an upcoming brake bleed is a spongy feel to the lever, indicating that air has managed to get into the system. This could happen when you cut a hose to shorten it, or if you crash and manage to rip one of the hoses from the lever or nick it elsewhere. Air is more compressible than the incompressible brake fluid, so when you pull the brake lever all the air bubbles will be squashed before the fluid starts to go anywhere. Thankfully air is lighter than brake fluid, so if you open the system at the top the bubbles will rise to the top and escape.
The process of opening the brake, letting the air out and topping up the fluid level before closing the brake again is known as bleeding. Bleeding is often regarded as a mysterious and complicated process only to be carried out by druids. Actually, it’s quite simple but people do tend to treat it as a universal cure for anything wrong with the brakes, whereas it should actually be one of the last things you consider unless you have good evidence that there’s air in the system.
You should also take care not to get hydraulic fluid of either variety anywhere near your rotors or pads. If you’re working with fluid, then remove the wheels and pads to get them well out of the way of any possible contamination. Do not refit them until you’ve finished the bleeding process, cleaned up and packed away. If the worst happens and you manage to contaminate the rotors, then you can try cleaning them with isopropyl alcohol but if the pads are contaminated, then they need to be replaced.
Open and closed system brakes
The vast majority of brakes currently on the market are open systems, rather than closed. Both types of brake operate in the same way, with a master cylinder at the lever operating a slave cylinder at the calliper. Under heavy braking, the fluid heats up and expands, increasing the volume of fluid in the system which would cause the pads to drag on the rotors if unregulated. This happens to both open and closed systems, but they differ in the way in which they deal with the heat build-up.
Open systems have a flexible rubber diaphragm inside the reservoir (usually at the top). As your brake fluid heats up and expands, the diaphragm deforms to accommodate the additional volume. The piston inside the lever is designed to close off the reservoir from the brake hose as soon as it’s operated. This is vital – if the hose and reservoir remained connected then the force of the lever would simply crush the diaphragm, rather than acting on the pistons. With a basic open system, you can’t adjust the pad position independently of the calliper, though some open brakes (like Hope’s X2 and Avid’s Elixir series) now include bite point adjustment.
Closed system brakes lack the diaphragm of open system brakes, so cannot auto-adjust for fluid expansion. They do however have an adjusting knob on the reservoir which allows you to dial the bite point of the brake in and out by changing the volume of fluid the reservoir can hold. Making the reservoir smaller by screwing the adjuster in forces fluid down the brake hose, pushing the pistons and pads towards the rotors. Making it larger, by screwing out the adjuster, causes the pistons and the pads to retract from the rotor. This means that under heavy braking, for example on a long alpine descent, you can back the pads right off to accommodate the fluid expansion and eliminate brake rub – the process which the closed system does automatically.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES