Shortly after commencing the taxi for takeoff, a hydraulic union in the braking system fractured, causing fluid to leak from the Yellow hydraulic system. The departure was cancelled and the aircraft returned to the terminal. After stopping on the allocated stand, the parking brake was selected on, but the brakes failed to apply, as the parking brake is operated by the Yellow hydraulic system. The aircraft then began to move forward under idle engine power...The aircraft collided with the airbridge, damaging the left engine inlet cowl, before coming to a stop.