Pressure off Brake A POB is attached to each hydraulic motor. It holds the output shaft of the hy-draulic motor when: - the hydraulic motors do not operate - the related hydraulic system does not supply sufficient,hydraulic power - the WTB stops the flap transmission system because of some system failures. The POB has a multiple friction-disk pack. Splines connect the stators to the POB casing and the rotors to the shaft in the center of the POB. Springs hold the friction disks together. When hydraulic pressure is applied to the POB, the friction disks are disengaged (against the pressure of the springs).To remove the POB, you must remove the related hydraulic motor first.