B737-800
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance; Flight Crew Returned To Gate; Flight Crew Returned To Departure Airport; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Landed in Emergency Condition; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft
Approximately ten minutes after reaching cruise altitude FL360 we saw the master caution flicker. No light on recall. 2 minutes after that we had a steady master caution with hyd (hydraulic) light on recall. At that point we determined that it was System B through illumination of the low pressure lights as well as falling system B pressure on Hydraulic page. We [requested priority handling] with Center and requested vectors back to ZZZ. Set up for the ILS XX direct [the initial fix] and the CA (Captain) went through the QRH as we headed back. Uneventful flaps 15 landing; greeted by Fire rescue and they followed us to [the gate].
The aircraft had a loss of system B hydraulic pressure at cruise altitude FL 360. System quantity and pressure showed 0. We were approximately 75 miles west of ZZZ1 on filed route. CA (Captain) was pilot monitoring and FO (First Officer) was pilot flying. We requested priority handling and returned to ZZZ while completing appropriate checklists. Aircraft landed and stopped on RWY XX. After a safety check from ARFF (Aircraft Rescue and Fire Fighting) we taxied back to gate with a fire truck escort as a safety measure.
B737-800 flight crew reported loss of hydraulic system during cruise.