Date: 201703
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Hydraulic Fluid
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed As Precaution; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: During van ride we noticed a write-up from the inbound crew with a defect of SYS A LOW PSI LIGHT ON DURING GEAR RETRACTION LIGHT EXTINGUISHED HYD A QTY STEADY 18PCT. When we arrived maintenance informed us they were looking into the write-up. At update time they were still working and the decision was made to move the aircraft off the gate so it could be used by another aircraft. The aircraft was towed [to the gate] and we were informed there was a leak by the solenoid valve gasket plate and it was repaired and system was serviced full. No issues during ground ops and lined up for takeoff. After putting the gear handle up we got the Master Caution light with HYD and FLT CNTL lights. Shortly thereafter the light went out and we noticed system A was 76%. System A continued to increase to 78% while we continued our climb. Started discussing the issue when we noticed system A quantity started dropping. Due to our knowledge of the write-up from the last flight and the confirmation from maintenance that there was a leak we decided to call Ops frequency to talk to maintenance. We expressed our concerns and decided since system A quantity continued to decrease we may have a leak. We then decided the safest course of action is to go back to [our departure airport]. We contacted ATC for a return to and asked the controller to pass to company. The Captain TEST'd (Type of Emergency; Estimated time available; Signal for brace; Take special instructions) the FAs and informed the passengers that we were going back to [the airport]. The Captain and I then discussed the fact we were heavier than max landing weight (we were at 160.0k at the time). As we discussed plans to get under the weight we got an ACARS message from Dispatch that [maintenance control] had already coordinated with maintenance for a heavy weight landing inspection if we chose to do it. We decided to land since our concern was a hydraulic leak which would get worse with time. We configured early and noticed the system A hydraulics jumped back into the mid-80s. The Captain flew a visual approach to the runway and rolled to the end. Emergency vehicles were standing by and the emergency was terminated.  We then entered crew rest.Talked to a maintainer who told us there was no leak but they did find air in the system which could have caused nonstandard indications. Took off and had no issues on [the next flight].


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported the 'A' hydraulic system quantity indication began to diminish in flight.