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 hydraulic (hydraulic) light. We continued the takeoff. After retracting the landing gear; the master caution illuminated again; this time with the hydraulic and flight 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.

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Original NASA ASRS Text

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

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.