What was found
PHX - AIR TURN BACK - FLT 0175 - EN ROUTE FROM PHX TO MEX IN CRUISE ECAM INDICATION FOR GREEN HYDRAULIC SYSTEM LOW LEVEL FOLLOWED BY ECAM HYDRAULIC LEVEL IN AMBER. ON RETURN TO PHX TURNED ON GREEN PUMP / PTU AUTO PER ECAM INSTRUCTIONS AND RECOVERED NORMAL PRESSURE. LOWERED LANDING GEAR AND RECOVERED 1/2 OF HYDRAULIC QUANTITY, NO FURTHER HYDRAULIC LOSS. LANDED SAFELY AT PHX, AIRCRAFT TOWED TO GATE. MAINTENANCE FOUND HYDRAULIC LEAK AT SLAT POWER CONTROL UNIT ON R/H SIDE LOWER FLEX LINE UNION. REPLACED O-RINGS ON FLEX LINE UNIONS. OPERATIONAL AND LEAK CHECK WAS GOOD PER 27-84-00-710-001. PART NAME; O-RING, MFG; NATIONAL AEROSPACE STANDARDS (NAS), P/N; NAS1612-6, COND; LEAKING, LOC; ZONE 140.