Brake bleeding is a procedure performed on your brake system where brake lines containing brake fluids are removed of any air bubbles. Over time air and moisture can get into your brake lines that can compromise your safety and the effectiveness of your brake system
How to check if you need brake bleeding:
Confirming that you need brake bleeding is important. The sinking brake pedal often does mean the brake lines need to be bled. However, it is very important to confirm that the sinking pedal is not caused by something else.
(These methods are common knowledge and not to be taken as instructions)
Try this simple test when you are stopped and waiting at a red light. With your foot, keep an even pressure on the brake pedal. Does the pedal sink lower, at all? If so, then you should have your car’s brake system inspected by an ASE Certified Master Auto Technician to confirm the root cause is not something else. If the pedal remains at the same height, then you may just need to remove air that has entered into the brake hydraulic system.
Sinking brake pedal could also be caused by other things that could be very dangerous. For example, brake pedals can also sink if there is a hydraulic problem, like a failing master cylinder, a leaking rear wheel cylinder, a bad caliper or bad ABS. So it is important to rule out these dangerous possibilities via professional inspection before proceeding.