alexban,
If you have a hydraulic leak on a B737 classic or NG the System B hyd reserviors will loose fluid down to the level of the standby standpipe located with-in the reservior(assuming the crew have allowed fluid to continue to leak past the standpipes located with-in each reservior for each system electric driven hyd pump).
The Stand pipe with-in system B hyd will save the fluid below the standpipe for operation of the Standby Hyd system.This system will provide hyd power to the standby rudder (also WTRIS system in the NG)and the thrust reversers,and leading edge flaps/slats.
The trailing edge flaps will run electrically.
All other flight control will be using manual reversion.
Also note that the ailreon system has an actuating Power Control unit for each hyd system,they use independant inputs and each are capable of operating the entire aileron system by themselves so the odds of the failure you mention are extreme to say the least.
Note also that wide body (& A320 narrow body jets) use totally hyd powered flight controls that utilize 3 to 4 seperate systems with multiple redundances.
Regards DDG