If you are driving along and then need to apply your brakes only to find that your brakes are hesitating or jerky, you may have air bubbles in the brake lines. For the most part, bubbles in the brake lines are just a nuisance, but if you have to apply your brakes suddenly and that is the moment that they do not work because of an air bubble, you could get into an accident. This is why you will want to "bleed" your brakes, or remove the air bubbles from the brake lines. There are four ways you (or an auto service technician, such as Discount Brake Center) might do this.
Vacuum or Suction Method
This method is probably the most common, because it does not take much to suction air out of the brake lines. The hoses that comprise the brakes may be directly connected to a syringe in a pinch, otherwise a special vacuum pump made for bleeding brakes is applied to the bleed port on each side. The vacuum pump has a hand trigger that, when squeezed, creates a vacuum in an attached chamber. You or the mechanic can open the port, attach the pump, and pull the trigger until you no longer see any air bubbles escaping into the vacuum chamber from the brake line port.
The Pump and Hold Method
The bleed screws on your brakes should be opened one at a time, not all at once, if this method is going to work. As one person opens each of the screws, another person pumps the brakes a couple of times, and then finally holds down the brake pedal. You will hear the air in the brake lines escaping. I may sound like a hissing or "burping" noise. Continue to pump and hold the brake pedal until you cannot hear any more air escaping from the bleed screws. Then you know the lines are free of air bubbles and your car no longer poses a potential accident risk.
The Pressure Method
A pressure pump with a manual, vertical hand pump is attached to the master cylinder to force a lot of air through the brake lines. All of the bleeder lines are opened and then you rapidly pump the handle on the pressure pump to force air through the lines. Air bubbles escape through the open lines. Then you or the mechanic can remove the pump and close all the lines back up again.
The Reverse Pressure Method
The same tool used in the pressure method is applied here, except that you do not connect the pump to the master cylinder. You connect it to the bleeder valves and force the air back out the master cylinder instead. This method is only good for use if you or your mechanic know beyond a reasonable doubt that the air bubbles are closer to the cylinder and not farther down in the lines.