First, there are many reasons, such as:
A-brakes are too hard.
B-brake wear level has been fixed.
The C-brake disc or brake hub is too worn.
The D-brake sub-pump is rusty.
E-hand killing the line is not alive.
F-killer master cylinder returned slowly.
Second, the possible situation and treatment:
1. If it is screaming, the first thing to check is that the brake pads are running out (the alarm piece sounds). If it is a new film, check to see if any foreign objects are caught between the brake disc and the sheet.
2. If it is muffled, it is mostly a problem with brake calipers, such as the wear of the movable pin, the spring piece falling off, and the like.
3. If it is silky, then there are more problems, calipers, brake discs, brake pads may have problems. If the sound is continuous, first check if there is a brake. If the caliper is not properly reset, it will cause the disc to rub against the sheet for a long time. Under certain conditions, an abnormal sound will be emitted. Then check if there is any foreign matter stuck between the film and the disc. If it is a new film, it depends on whether the brake disc has a groove, because local friction will cause abnormal noise. If the disc is ok, consider whether the brake pads are too hard or the muffler is installed incorrectly.