So, first of all, the hydraulic pressure gauges require generator power to function. You can test this by turning off both generators in flight and the hydraulic gauges will drop to zero despite the engines still being on and providing hydraulic pressure. The Gens don't provide enough voltage while just windmilling and so the pressure reading also drops to zero.
There's an electrically driven pump that is supposed to provide some hydraulic pressure to the PC-1 side of the Stabilator in the event that hydraulic pressure drops below about 1000 psi.
Even then, if you are in the air, it is likely the engines are windmilling and providing some hydraulic pressure through that avenue as well.
Rudder control is still possible even without hydraulic pressure.
