What was found
INFLIGHT MSP-PHX, CREW RECEIVED 'HYD Y SYS LO LVL' ECAM MESSAGE. IT WAS DETERMINED THAT YELLOW HYDRAULIC SYSTEM WAS LOST. AN EMERGENCY WAS DECLARED AND AIRCRAFT LANDED WITHOUT INCIDENT IN PHX AND TAXIED TO GATE UNDER ITS OWN POWER. MAINTENANCE REMOVED AND REPLACED PARKING BRAKE VALVE, 2579GM, IAW AIRBUS MAINTENANCE MANUAL 32-45-14-000-001-A.