Disc brakes are essential to safely descending on a mountain bike. When they work, they have superior modulation and stopping power, and are considered the top of the line mountain bike brake system. However, like any braking system, they rely on pads, usually made out of a metallic or resin-based compound, for their stopping power. When these pads get worn down or contaminated, usually from oil on the pads, their stopping power suffers. A few simple steps will help diagnose disc brake stopping issues.
Flip open and unscrew the quick release on the wheel of the affected disc brake system, then remove the wheel from the drop outs. Look inside of the disc brake caliper, which is the mechanism that holds the brake pads and is attached to the bottom of the fork or frame, depending on the wheel.
Check the amount of material on the brake pads. There should be about a millimetre or more of pad material on the metal pad backing. If there is less than that, then it is time to have the pads replaced by a professional bicycle mechanic.
Rub sandpaper on the pads. Insert a piece of sandpaper into the caliper and run it back and forth along the pads about 10 times in order to shave off a small layer of material. Then repeat the step for the other side of the pads.
Spray acetone on a clean rag, and wipe the rag along the disc brake rotor, which is the metallic disc on the bicycle's wheel. This will get any oil or contaminants off the rotor. If you have acetone in a spray bottle, spray some inside of the caliper to remove any contaminants in the caliper.
Insert the wheel back into the drop puts, and spin the locking nut on the quick release until it barely touches the bicycle frame. Then flip the quick release lever closed. Apply the brakes while riding at low speed to see if the stopping power has improved.
If none of these steps work, have a local bicycle retailer replace the pads. Pay attention to what kind of chain oil you use and how you lubricate your chain. Try not to use oil in a spray can, as that can spray onto the disc brake rotors. If you use oil in a drip bottle, make sure not to accidentally get oil on the rotor.
Make sure not to touch your rotors with bare skin, as the oils on your hands can contaminate the rotor. It is best to use rubber gloves when working on the rotor.