B737 first officer reports receiving an aircraft from maintenance after an 'a' check. At FL350 hydra...

Narrative:

After a 2 hour delay for aircraft to be placed in service after an overnight 'a' check. Maintenance technician was rebalancing hydraulic a and B fluid levels. It was replenished to 95%. After reaching cruise flight at FL350 I began to monitor hydraulic system quantity and pressure. I alerted captain to a level at approximately 74% and started to drop progressively 1-2% in the next several minutes. Captain gave aircraft flying duties to myself and contacted company maintenance and dispatch via radio. After 2 separate conversations with company and upon reaching the quantity level of approximately 62% on a hydraulic system; the captain felt the safest course of action was an air return to the departure airport. I agreed. It afforded us the longest runway available and premier crash/fire rescue services. We reviewed QRH and briefed flight attendants/passengers/cockpit jumpseat rider. Captain assumed flying duties and I resumed pilot monitoring duties. An emergency was declared and ATC cleared us back direct. Minimal aircraft maneuvering was requested. A flaps 30 autobrakes 3 landing was performed without incident and crash/fire rescue advised no exterior negative mechanical indications were observed. We taxied to gate and deplaned passengers and affected a logbook entry as to the aircraft condition.

Original NASA ASRS Text

Title: B737 First Officer reports receiving an aircraft from Maintenance after an 'A' check. At FL350 Hydraulic System A quantity is noted to be solely decreasing and crew elects to return the departure airport.

Narrative: After a 2 hour delay for aircraft to be placed in service after an overnight 'A' check. Maintenance Technician was rebalancing hydraulic A and B fluid levels. It was replenished to 95%. After reaching cruise flight at FL350 I began to monitor Hydraulic System quantity and pressure. I alerted Captain to A level at approximately 74% and started to drop progressively 1-2% in the next several minutes. Captain gave aircraft flying duties to myself and contacted company Maintenance and Dispatch via radio. After 2 separate conversations with company and upon reaching the quantity level of approximately 62% on A Hydraulic system; the Captain felt the safest course of action was an air return to the departure airport. I agreed. It afforded us the longest runway available and premier Crash/Fire Rescue services. We reviewed QRH and briefed flight attendants/passengers/Cockpit Jumpseat Rider. Captain assumed flying duties and I resumed pilot monitoring duties. An emergency was declared and ATC cleared us back direct. Minimal aircraft maneuvering was requested. A flaps 30 autobrakes 3 landing was performed without incident and Crash/Fire Rescue advised no exterior negative mechanical indications were observed. We taxied to gate and deplaned passengers and affected a logbook entry as to the aircraft condition.

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.

Attributes
ACN	1178845
Time
Date	201406
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Hydraulic Main System
Person 1
Function	First Officer
Events
Anomaly	Aircraft Equipment Problem Less Severe

B737 first officer reports receiving an aircraft from maintenance after an 'a' check. At FL350 hydraulic system a quantity is noted to be solely decreasing and crew elects to return the departure airport.