Date: 201909
Local time of day: 1201-1800
Location: DEN.Airport, CO
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
Airspace: Class B DEN
ATC / advisory: Tower DEN
Aircraft component: Hydraulic System
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: Taxi
Result: Air Traffic Control Provided Assistance; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: After landing and while we were taxiing to the gate; we noticed a problem with aircraft control. We had completed the after landing flow and were taxiing southbound towards the terminal. I noticed that the plane was veering left and right from the taxi centerline and was getting harder and harder to correct back to center. The First Officer was on Radio 2; calling ramp control to verify that our gate was open; so I pushed the Hydraulic Systems page button and saw that the System 'A' Hydraulic was showing 0000psi; and '00-RF' quantity. I brought the aircraft to a stop abeam a taxiway in order to trouble shoot the problem. We notified Ground Control that we needed to stop; and they said that was approved for that position. The First Officer pulled out the QRH checklist for loss of system A hydraulics. We ran through the checklist procedure. I then switched to radio 2 to call maintenance in Denver; while the First Officer monitored Ground Control on radio 1.At that point we noticed system 'B' hydraulic pressure was dropping. [Another aircraft] was taxiing past us on and said he saw that we were leaking fuel and he thought he saw smoke. The First Officer got on the radio and said it was hydraulic fluid. At this point I called the flight attendants and told them we were having an issue and to standby for [details]. The system 'B' hydraulic quantity dropped to 15% and then held there at the stand pipe level. An airport operations vehicle approached our aircraft and contacted us through the ground control frequency. We communicated with him our problem. At this point the airport fire department trucks started to show up. (I assume because of the report of light leaking fluid and smoke). We decided to shut down the engines at this point; and started the APU; and ran the Parking Checklist.The system 'B' fluid was maintaining at 15% to 20%. We decided to run through the 'Manual Reversion or loss of system A and B' checklist anyway. At this point we did turn off all hydraulic pumps. We called the flight attendants and told them the situation; and said fire trucks would be approaching the aircraft. I made a PA to the passengers; notifying them that we had a system malfunction and that fire trucks would be inspecting the aircraft. Not to be concerned; that it was only a precaution. But; we would likely need to be towed to the gate. The fire department confirmed it appeared to be hydraulic fluid coming from the left main gear area; and we coordinated with them; for chalking the aircraft in place; in case we lost the standpipe fluid holding the parking brake pressure. We coordinated with airport operations and company operations for a super tug to retrieve the aircraft. We submitted three electronic logbook reports: Hydraulic fluid leakage. Hydraulic fluid quantity indication shows refill system A. Hydraulic fluid quantity indication shows refill system B. The super tug towed us to our gate. We ran the parking check list again and deplaned normally.


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


Synopsis: B737 flight crew reported a loss of hydraulic pressure while taxiing after landing.