Just to correct a misleading impression in the preceding post. From Part B (Procedures):
The QRH states "If only one hydraulic system remains, LAND AT
NEAREST SUITABLE AIRPORT'. The B737 has three hydraulic
systems "System A", "System B", and the "Standby System".
Boeing consider only one hydraulic system remains following:
Loss of both System A and System B; or
Loss of System A and the Standby System; or
Loss of System B and the Standby System
Loss of any one system would, I think in most circumstances, lead to the following: outbound- return to base (no point going if you will be stuck away). Inbound- continue to base. If on single multi leg trip- review most efficient maintenance option. Always review MEL.
The point is- loss of a single system in a 3 system aeroplane does not cause an emergency response. There are no specific diversion instructions relating to loss of a single system.