For 1990-2009 cars

Caution: Do not move the vehicle until a firm brake pedal is obtained. Air in the brake system can cause the loss of brakes with possible personal injury.

Whenever air gets into the brake hydraulic system, you must bleed the system to remove it. You might have to bleed the hydraulic system at all four wheel positions if air has entered the system due to low fluid level, or from brake pipes having been disconnected at the master cylinder. If a brake pipe is disconnected at one wheel, only that wheel cylinder/caliper needs to be bled. If pipes are disconnected at any fitting located between the master cylinder and the wheels, then the brake system served by the disconnected pipe must be bled. Refer to hydraulic system bleeding.