my question comes from the A320 specifically, and I also suppose Boeing jets. Whenever you check the hydraulic system pressures on an A320 it says 3,000 psi, even when you move the flight controls. How is this pressure a constant 3,000 psi no matter what? What is creating the resistance to create this pressure? Thank you very much!
The hydraulic system contains a pressure regulator device which maintains almost the same source pressure to the cylinders across the full range of flow rate demands, regardless of what the pressure being developed by the pump might be.
In practice this would mean designing the pump to develop a minimum of, say, 3100 PSI across all engine speeds and then putting in a pressure bypass that automatically flows the extra 100 PSI back to the pump sump.
When the flow demand on the hydraulic system is high, the pressure bypass progressively closes so as to maintain 3000 PSI throughout the system.
Another way to do this is to control the pump motor with a pressure sensor that automatically cycles the pump motor anytime the system pressure falls below 3000 PSI. What you will hear in times of high hydraulic system demand is a surprisingly loud VOOOOT! VOOOOT! VOOOOT! noise as the pump cycles on and off to hold the source at 3000 +/- (some value) PSI, which I have indeed experienced on the 320 upon landing (when the flaps, spoilers and whatnot are all being retracted).