A319 Captain reported the flight crew received a Master Caution and ECAM for HYD SYS LO PRSR for the Yellow Hydraulic System. Subsequently; a FLT CNTRL ECAM was received due a partial spoiler deployment (#4) on the right wing. The flight crew requested priority handling and diverted to make a precautionary landing.

Narrative

Departed ZZZ; Aircraft X; to ZZZ1 on Date. Approximately 100 E/NE ZZZ2 while at FL340 at XA50Z; while deviating north for weather; the flight crew received a Master Caution and ECAM for HYD SYS LO PRSR for the Yellow Hydraulic System. Subsequently; a FLT CNTRL ECAM was received due a partial spoiler deployment (#4) on the right wing. The First Officer was the Pilot Flying and the Captain was the Pilot Monitoring. All QRC; ECAM; QRH and follow-up procedures were completed by the Pilot Monitoring. While the Pilot Flying continued the weather deviation; the Pilot Monitoring conferred with dispatch on flight options and diversion details; to include the NOTAMs for ZZZ2. A request for priority handling was made and a diversion to ZZZ2 with ATC priority radar vectors for descent. Coordination via a precautionary landing brief to the flight attendants and passenger PA were also completed. Additional information regarding the partial spoiler deployment was obtained via communication between the Pilot Monitoring and the number one Flight Attendant via a visual inspection of the right wing. There was a noticeable right wing dip at altitude and increased buffet and droop during the descent and approach phase of flight. The Pilot Flying executed a visual approach; with the electronic means of backup using the ILS XXR to an underweight landing (132;000 pounds). After clearing the Runway; an Airport Rescue and Firefighting vision inspection was executed; with the crew confirming flight crew suspicions of hydraulic fluid loss along the belly of the aircraft. With the Green; Blue hydraulic systems intact; the aircraft was taxied to Gate XX. Cause was determined to be the mechanical failure of Yellow System Reservoir. ECAM status page mentions approach phase procedures which result in return of HYD SYS LO PRSR ECAM warning (engine pump on) which can confuse with electric pump on since both work with Yellow System.