From my copy of A320 FCOM:
Quote: Priority valves cut off hydraulic power to heavy load users if hydraulic pressure in a system gets low.
and A330 FCOM:
In the event of low hydraulic pressure, a priority valve cuts off hydraulic power to heavy load users (emergency generator, nose-wheel steering, landing gear) in order to keep the pressure for normal braking and flight controls.
A Pressure-Off Brake system (installed on the flaps, slats, and THS actuator) ensures the same function.