Date: 201901
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight phase: Takeoff / Launch
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Hydraulic System Pump
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: On takeoff; First Officer was Pilot Flying. As soon as the gear was raised; we were presented with all the indications of an 'A' system hydraulic failure or low pressure. Master caution; eng driven pump low press; flt cont A low press; stby rudder on; etc. Hydraulic system page indicated 76% quantity (ok); pressure 1700 psi but slowly rising. Landing gear came up slower than normal; but once up and locked; pressure returned to normal of 3000 psi. We monitored the system and determined there was no loss of fluid and good pressure. We elected to continue to [departure airport].We anticipated when we lowered the gear for landing we may see a decrease in pressure. We lowered the early and pressure dropped to 2300 then came back up. We anticipated the left reverser may also be slow and it was. I wrote up the discrepancy in the [maintenance log] and gave it to Maintenance with a thorough debrief. I told them we would not accept the aircraft back without repairs. It appears likely the engine driven pump was not producing sufficient pressure and was being assisted by the electric driven pump and standby system.This is an old aircraft that I believe experiencing signs of old age. The pump was getting ready to fail. We reviewed the relevant QRH procedures in the event of failure but system quantity and pressure were normal after the high load was removed. I wrote this [report] to highlight what I see as an anecdotal trend with the older 737 airframes. This was the second aircraft in recent days I had been presented with that had developed significant maintenance issues. The other was four days prior where we picked up another aircraft that pulled into the gate with the wing fuel tanks burning a significant amount of fuel before the center tank fuel began to be consumed. Likely; the 'old' center tank pumps were not putting out nearly enough pressure to operate properly and empty the center tank FIRST. I also made it clear to Maintenance that I would not accept that aircraft. To their credit; in both cases; Maintenance didn't try to dissuade me and found us a replacement aircraft promptly. My conclusion is these older 737s need more attention from Maintenance for dynamic components like hydraulic and fuel pumps that are reaching the end of their life cycle. I hope this will heighten awareness of this issue.




Synopsis: B737-800 Captain reported the output of the A hydraulic system pump seemed below normal; and  that the aging fleet will need more maintenance attention.