B737-800
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Less Severe
Flight Crew Diverted; General Declared Emergency
Aircraft; Human Factors
Flight Control B Low Pressure Light illuminated in cruise. Followed QRH procedure and placed Flight Control B Switch to STBY RUD. B system quantity and pressure were normal.  While autopilot A was engaged at the time; there were no apparent flight control issues; although there was increased oscillation in yaw. The procedure offers no additional steps or information. When we placed the switch in STBY RUD; the Feel Diff Press; Stby Rud On and Yaw Damp lights illuminated; which seemed appropriate for the procedure but were not mentioned in the QRH. I consulted Dispatch and Maintenance Control. Maintenance Control advised that flight control operation would be normal; however we would not have use of 2 flight spoilers. This is not mentioned in the QRH. Based upon this information and the increased yaw oscillation; we declared an emergency and diverted. With the anticipation of possible extra braking being needed; we requested ARFF meet the aircraft. Landed without incident. When ARFF advised that the brakes appeared normal; we terminated the emergency and taxied to the gate without further incident.
AT cruise (FL 310) B system flight control low pressure light came on. Completed flight control low pressure light checklist and diverted to ZZZ1 (Destination was ZZZ).Checklist provided no guidance regarding degradation of systems. Maintenance Control informed Captain that we would loose 'some of our spoilers.' Captain elected to use a longer runway based upon this input. Recommend specifying in the checklist that a normal landing is indicated in this situation if that is the intent.
B737-800 crew reports a Flight Control B Low Pressure Light illuminated in cruise and QRH procedure required placing Flight Control B Switch to STBY RUD.  After consulting with Maintenance Control crew elects to divert to an airport with a longer runway.