Date: 201811
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: Mixed
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Airspace: Class C ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Hydraulic System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: We received the aircraft in ZZZ with a heads up from the Ops Agent that the aircraft had experienced a hydraulic leak the night before and that Maintenance had worked on it. The Captain and I looked over the logbook which showed that after doing systems checks; Maintenance did not find the source of the leak; therefore no fix or repair was made; but fluid was refilled and 'Ops check good.' The Captain and I each performed a preflight inspection; ran the hydraulic pumps; and did not see a hydraulic leak. As part of the brief; the Captain reviewed from the checklist which systems would be affected if we had complete system A fluid loss or failure.I wrote down the fluid levels in each system; both A and B showed 97%. The Captain was PF (pilot Flying) and I was PM (Pilot monitoring). Engine start up and taxi out to runway XX were uneventful. During takeoff after 80 knots; we both saw a momentary flash of the Master Caution HYD (Hydraulic) light. We continued the takeoff. After retracting the landing gear; the Master Caution illuminated again; this time with the HYD and FLT CONT (Flight Control) annunciators. On the overhead panel; the A Flight Control Low Pressure light and the STBY RUD (Standby Rudder) On light illuminated. These lights were only momentarily illuminated and extinguished soon after we noticed them.The autopilot kicked off and the Captain could not re-engage it for a few moments. Sometime after we noticed the annunciators went out the autopilot was able to be re-engaged. At that point the hydraulic fluid level for system A was now in the 70s; so we decided to return back to ZZZ. I told ATC we wanted to return; requested vectors to the ILS for the longer Runway XY. On downwind; the Captain decided to extend the gear early. We had approximately 67% fluid at that time and wanted to minimize our workload in case more fluid was lost or in case the system failed. After the gear was extended; the fluid level showed back in the 70s.We proceeded with the climb; Descent; and Approach Checklists. The Captain made an announcement to the passengers. I got the landing numbers and got the aircraft instruments set up for the approach; and contacted Ops on frequency to let them know that we were returning to land and would be on the ground shortly. In the interest of time; and managing the situation safely; we did not notify Dispatch that we were returning to ZZZ; but kept ATC up to date with our plan and requests. We did not run any Non-Normal checklists and did not declare an emergency as every system was still operating properly. After landing; the flaps retracted normally and System A fluid level showed 65RF. Taxi back to the gate was uneventful. Flight Attendants reported no issues with passengers; and mentioned everyone was able to get off the aircraft without any hassles.It seems the problem was not properly addressed the night before. The previous crew reported fluid loss beyond dispatch-able levels and hydraulic fluid in the wheel wells; yet the hydraulic fluid was simply re-filled. More investigative work should have been completed before this aircraft was returned to service.


Second narrative: We arrived at our originating Aircraft X in ZZZ. During preflight; we reviewed logbook and noted a writeup on the terminating flight. That pilot said that A side hydraulic had a leak and the quantity was at 25 percent. The mechanic that worked on the aircraft wrote that all A systems were checked and that no leak noted. He then serviced the quantity and signed off aircraft. We had uneventful taxi and were cleared to take off Runway XX. After 80 knots; we got a momentary Caution light (HYD). The takeoff was continued. At gear retraction; we got Master Caution (HYD) and Overhead FLT Control A Hyd Low. This light stayed on for approximately 15 seconds then extinguished.That light nor any other alert came back on. We continued the climb to 4;000 feet and assessed. We noticed that HYD A quantity was decreasing. I made the decision to return to ZZZ. This was based on the previously writeup coupled with our alerts and decreasing HYD fluid quantity. We were only 115;000 pounds at this time. We asked Tower for vectors to XY and started the return. Because we were losing fluid on the A side; we lowered the landing gear early in case we had a total depletion. There were no system malfunctions; therefore no NNC (Non-Normal Checklist) to run. The First Officer retrieved the landing numbers then we completed all normal checklists.FO (First Officer) called ZZZ Operations to tell them of our return and I made an announcement to the Flight Attendants and passengers. No emergency was declared; however I did use my Captain's emergency authority when I decided that in the interest of time and the Safety of the flight close to the ground that we would not have time to contact Dispatch to change the destination airport. We made a normal landing and all systems worked properly all the way to the gate. The A side Hydraulic quantity at engine start was 97 percent and at shutdown it was 65%. The mechanic then located a leak in the left wheel well. No passengers complained and Cabin Crew was debriefed.I question the maintenance procedure that was performed the night before as to its effectiveness. The aircraft was signed off with no leak found even though the A quantity was down to 25%.


Synopsis: B737-700 flight crew reported hydraulic quantity decreasing on climbout resulting in a return to departure airport.