An Air Canada Airbus A330-300, registration C-GEFA performing flight AC-7204 from Montreal,QC (Canada) to Zurich (Switzerland), was enroute at FL370 about 60nm west of St. John's,NL (Canada) prior to the ETOPS entry when the crew received a yellow hydraulic system low pressure warning. The crew consulted with maintenance and decided to return to Montreal. The aircraft climbed to FL380 for the way back and landed safely back in Montreal about 3:50 hours after departure.
The Canadian TSB reported maintenance installed a new hydraulic actuator and bled the yellow system of air.