Your levers can easily get 'clogged up' with dirt. When applied, levers should always move smoothly and noiseless. Checking your brake levers is easy - simply squeeze them.
When your gears are fully engaged, there should be approximately one inch of space between the inside edge of the lever and your handlebars. At this point your brakes should hold solidly against your full weight.
Levers may also slip out of position on your handlebars. The levers should be mounted firmly but with a bit of give so that in the event of a severe knock, they will move a bit rather than snap which could happen if they were mounted too firmly.
These include the brake arms, brake shoes (which house the brake pads) and the pads themselves. They can be set-up incorrectly or jarred out of position as a result of an accident or crash. They should be checked to make sure all parts move freely and are positioned properly.
Visually inspect both the front and the rear brake units and make sure they're centered on each wheel with the brake pads equidistant from the rim surfaces.
Note: Wheels that are out of true can cause many brake assembly problems.
Pads wear down therefore requiring more effort to fully engage the brakes and give slower response times. Pads can also be jarred out of position. Check regularly to see if your pads are glazed, or significantly worn. Glazed pads can be cleaned. Pads that are worn down significantly should be replaced. Unevenly worn pads can either be sanded or filed flat or may even need replacing.
Note: Each pad should fully contact the rim when the brake is engaged. They should do this without touching the tyre or hanging over the lower edge of the rim. Pads should be "toed-in" slightly when viewed from above. This is so the leading edge of the pad makes contact with the rim surface slightly before the back edge does. This improves brake effectiveness and prevents that squealing sound.
Brake cables and housings
Cables fray, rust and weaken over time. They also wear, kink and fray as a result of normal use. Cable housings that protect your cables can break, corrode or clog up over time. Cables should always move freely through all cable housings and the guides that hold them in place. Check your brake cables and housings regularly for visible frays, rust, or signs of wear during all major brake overhauls and maintenance checks.