Date: 202011
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
Aircraft component: Hydraulic Main System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Procedure
Primary problem: Procedure

Narrative: Change in maintenance procedures after servicing or replacement of parts in the hydraulic systems.I was flying with LCA (name deleted) for my Latin America route qualification; to finish the Captain Upgrade syllabus.   We got a late start because of our gate being occupied by another aircraft experiencing maintenance issues.   Once we received our airplane X and performed all associated preflight flows; briefings; and checklists we pushed about an hour behind schedule.  I noticed that a leaky valve on the Yellow hydraulic system was replaced and a RON check had been performed that morning.  Taxi; take off from runway XX and the initial climb up through FL180 was normal.   Somewhere around FL200 the ECAM flashed a message; too quick for us to see what it was; about 30 seconds later it flashed again; and then another 2-3 times before the ECAM Message stayed displayed; hydraulic Y RSVR LOW LVL.  I was Pilot Flying; and called for ECAM actions.  LCA (name deleted) performed the ECAM actions and then consulted the COM for further instruction.  At this time we were nearing our cruise altitude of FL330.  I remained in control of the aircraft and ATC communications while LCA (name deleted) contacted maintenance through ACARS about our issue.  Dispatch and Maintenance both confirmed that a return back to ZZZ was the best course of action.   I requested a return to ZZZ with ZZZ1 center and stated we had hydraulic issues.   Center Cleared us back to ZZZ via direct ZZZ and the ZZZ Arrival.  The next communication with ZZZ1 I declared an emergency to get services in place for our arrival.   I proceeded to load the MCDU for our return to ZZZ.  LCA (name deleted) kept in communication with Maintenance and Dispatch during this time.  LCA (name deleted) also gave the TEST briefing to the lead Flight Attendant (FA); and made all PA's in both English and Spanish to the passengers.  Once we were established back towards ZZZ and we were caught up on our duties; I briefed the arrival and approach; we both discussed outcomes of the landing to be prepared for anything that may arise with our associated failure.  The loss of the Yellow hydraulic system would require us to bring the airplane to a complete stop on the runway because of the loss of Nose Wheel Steering.  LCA (name deleted) calculated a landing distance assessment; and went through the procedure for an overweight landing.   LCA (name deleted) took over control of the aircraft on the arrival because he has been flying recently and I haven't had a landing in the plane in 3.5 months; My most recent experience was a warm up and PC the week prior.  The arrival; approach; and landing were normal.  We landed and stopped on the runway XX; called for 'flight attendant stations'; Crash fire rescue inspected the exterior of the aircraft; and confirmed no hot spots.  Once given the all clear; the 'remain seated' announcement was made; we completed the landing checklist; shut down both engines with the APU running so the ground crew could hook up and tow the aircraft to the gate.   Once hooked up we were towed to Gate XX where we then deplaned and talked with maintenance about our issue.I believe the RA's functioned abnormally due to their antenna's prolonged exposure to snowy and icy conditions on the runway; taxiway; and ramp at ZZZ airport. I suggest an advisory could be sent out to keep the crews from being startled when this occurs in the conditions explained above. However; remind the aircrews to still comply with the terrain warning; whether it's an erroneous warning or not.




Synopsis: Captain reported that after a routine hydraulic valve replacement; the aircraft lost hydraulic fluid on climb out and was forced to turn back and perform a precautionary emergency landing.