The hydraulic brake system must be free of air to operate properly. Air can enter the system when hydraulic parts are disconnected for servicing or replacement, or when the fluid level in the master cylinder reservoirs is very low. Air in the system will give the brake pedal a spongy feeling upon application.
The quickest and easiest of the two ways for system bleeding is the pressure method, but special equipment is needed to externally pressurize the hydraulic system. The other, more commonly used method of brake bleeding is done manually.