Redundancy is the name of the game.
To: alexban
Good design practice dictates that the hydraulic system have built in redundancy. Most large civil aircraft have three hydraulic systems and some have electrically driven pumps for backup and still more have the ability to power up one system from another using a series of isolation valves that can be commanded to open and drive a hydraulic motor which in turn drives a hydraulic pump. This usually works in the event of an engine driven pump failure but will not function if the system has lost all of its’ fluid.
In the situation you described the aileron servo would have to suffer multiple structural failures in order to lose the two different systems that supply pressure. In the event of fluid loss the reservoir will isolate itself when the fluid level gets to a specified point.
It is difficult to conceive of a total loss of hydraulics under normal conditions. However if there is some external force such as that on the United MD-11 in Chicago that had an engine tear loose and fracture the hydraulic lines from all three systems. (Or, was it a DC-10)? The United DC-10 in Sioux City had an uncontained engine failure, which cut all three hydraulic lines. (Or, was it a MD-11)?
Forgive me, I’m 73 years old, and my memory is fading fast.